I'd say an iCandy Peach would probably be the best because it's got a small footprint.

P&Ts and the BJCS are very long, so would be a lot more awkward on a London bus. I tried out the City Select with my daughter and niece who were then 8.5kg and 10kg and it felt quite heavy to push and steer.

The new Phil and Teds Promenade could also work well, but is only out this month and there might be teething issues with it to start off with. The Kids Kargo Duelette is also a possible option and is much cheaper than the others.

Have a look here to compare sizes of different tandems to see which might suit you best.

The Peach would be no good because of the reclining lower seat given your two are over a year old. I think you would find the City Select heavy. Depending on where you are, it may well be impossible to use the tube if the access is not pushchair friendly. Most doubles should fit on a bus. The iCandy Pear is probably the smallest footprint in a tandem but manoeuvrability is difficult with older children on board. I think I would suggest looking at the Britax B-Dual, the Hauck Duett, the Kidz Kargo Duellette, the Mountain Buggy +One and Duet. The +One may be a very good option for you given your children are older.

Also look at the Joovy Caboose and similar sit and stand options.

I had a Phil and Teds Sport which I really liked, and which worked well for us until the older child was three or so (and able to walk).

It fits on the buses well and is easy to manoeuvre round shops but is very heavy if you happen to be at a tube station without a lift. If I knew I was going to be taking the stairs I would take a single buggy with a buggy board and make the older child walk on the stairs, which was easier to manage.

You probably want a single umbrella fold AND a double, ideally.

I had friends who had side-by-side doubles and they are just about doable but you have to get on the bus via the rear doors, which can be awkward, and if there is another buggy (or even a shopping trolley) in the space then you are stuffed. A top-and-bottom is definitely more practical and your older child will be out of it before long so you might also want to look at the best resale value (unless you think you will have more!)

By the way, you will usually find weights for strollers and car seats given in kg over here, so don't be afraid to quote your children's weight in kg in shops etc. Officially we are a metric country!

The B-Dual and Duellette are both similar like you say to the Phil&Teds. But the B-Dual cleverly uses height without adding to it. So the upper seat is much higher than a Phil&Teds one which gives loads of room for the child underneath. The basket on the B-Dual is huge and an older toddler can stretch their legs right out and still not reach the end of the basket. With the Phil&Teds basket space is always compromised. The Hauck is very similar but a bit cheaper product, with a harder ride. The B-Dual is quite a bouncy ride and the new 2012 version has even better wheels. The B-Dual however is not higher than say a Phil&Teds but just makes better use of space.
The Duellette gives a really nice option to have the children facing each other (as well as both facing forwards). I would say the seats aren't as big as the B-Dual or Duett but the children sit in them really well and it was easy to push. There's a large basket too.
Out of them all the B-Dual is probably the best quality, with best fabrics, brake, suspension, basket etc but I am not a fan of the way children sit in the seats - they just dont look comfy.
The Duett has a fab upper seat. The lower one is a little exposed - I dont like that there is no side support and my baby used to sleep sideways. Its a harder ride as I said but you wouldn't notice if you hadn't tried the B-Dual. The basket is good and all round I liked it.
The Duellette for me wins on seating positions. Lots of room, lots of options. I worry a bit about the build quality, but from what I have seen its good. How it (or even the Hauck) lasts long term I dont know. Also the seats are on the small side although the new version will have one higher seat.
Have a look at this for the B-Dual and Hauck Duett, this for some relative sizes, and this for relative sizes, and this for the Duellette bearing in mind the children are in winter gear!

Please dont get the Joovy Ergo Caboose - I think I can safely say it is the worst tandem I have ever tried! Its got a mind of its own, no steering, is heavy and unbelievably long!!! It looks short in the photos! These are lined up in length order; This might help?; or this??
A child certainly wont be able to nap in the back and the front Ergo seat is horribly hard and its nigh impossible at present to import a seat pad.

The Peach has a history - the early ones had very short straps which doesn't help things because it makes a child appear to not fit. You can get extension straps for these but not for the crotch strap, which helps a little but doesn't solve the issue. Equally people don't realise that the Peach hood slides up the frame, and it keeps sliding up if the zip is undone at the back to a very tall height!!! So height isn't an issue. Width shouldn't be an issue either as the seat liner removes easily if necessary (my 1m tall 3 year old still has it in). Leg length is more of an issue but the footrest rotates down to give more room, and if the child prefers, the Peach Blossom is high enough for feet to dangle. Its one of the biggest pushchair seats I know of. The lower seat is more of an issue because it is smaller and only reclines. But the new Peach Blossom will be out this year but thats too late for you. The new Peach doesn't have a bumper bar on the lower seat, and its simple to remove it on the current version.

No, iCandy don't recommend that you fit a buggy board to any of their products as the back axles are not designed to carry the weight of a child.

I think the Peach is fantastic as a single. Whether your youngest will be happy semi reclined in the lower Blossom Kit, I dont know. I know my children prefer to be upright. If it is an issue then the Pear could well be better for you, but its not perfect.

The B-Dual is almost as long as the BJCS - in fact I would say it was a tad longer to be honest. The thing that makes the difference though is how the space is used. The weight is better distributed and it takes full advantage of the available height.

I think you will struggle to find a neat tandem - there is the Mutsy Duo which is very neat - but its a nightmare to push and I am honestly not joking when I say the turning circle is something like 3.5metres. I could not envisage ever attempting to get it on a bus. Have you looked at the Mountain Buggy +One? If you are considering a buggy board then it may be a possibility. The rear seat is bolt right so not good for an older child to nap. The front seat has a slight recline. I have been trying out head supports for a rear child but haven't found anything that works as yet - I have one thing left to try. But its shortish and neat and if you can get through the next 6 months then it could well work long term for you.

The Mutsy Duo Seat can be put on the 4Rider chassis (it's on their website as a 4Rider accessory), but that doesn't change the fact that the front seat is so upright. Mutsy make a buggboard called the Step Up Board to go with the 4Rider. I'm not sure the upright front seat would work for you though.

I have a B-Dual and I'm not sure I'd want to use it on public transport in London to be honest. It's quite wide at 69cm as well as being a bit long (it doesn't feel too long to me though). But the seats are both a reasonable size and easy to access. And it's cheap!

Someone needs to build a true double decker pushchair :)

They do! Tiggy, what about the Kids Kargo Duelette?

I have mentioned that one above :)

kids cargo duelette ... how is it size wise? I think I might get this one but I want to know if you think its manageable around london/shops/buses, etc

EVERYONE round my way has a P&T of one type or another. You very very occasionally see a BJCS and I've seen one iCandy but I see dozens of P&Ts every day and have one myself. Heavy to handle, works absolutely fine on the bus but I only use it on a train/tube when I know I will not have to do stairs or if I've got another adult travelling with me (would use a single+ buggy board otherwise). I bought mine on ebay for a couple of hundred quid. It's a bit dog-eared but has been an essential item for me.

p.s. I use public transport more-or-less every day.

I would suggest the +One is very difficult for a child to sleep in. Yes I have seen photos of children sleeping in it, but its like them being in a square box ie straight sides. There are sides though and I have been trying head supports out and if you could find something like that, or even a pillow tied to the chassis or similar then it would be possible to make a child more comfortable, but it would take a bit of experimenting.

There's no doubt the Peach is better and easier to use than the Pear IMHO but its still much better than most other pushchairs on the market. In an enclosed space eg shop, the Pear feels much heavier but when you push it, it feels just like the Peach. But its not as good in tight spaces and I found the front wheels got stuck which is apparently down to the child weight. But it was a bit annoying. This might be useful but bear in mind it is the new Peach Blossom.

I can't comment on the Duellette as we dont use public transport. But it handles well and I loved having the children facing each other. It works better than the City Select for this.

Yes I have had one and Frigg has one.
But you said your eldest naps and the Rumble seat is worse than the +One for the napping!! The rumble seat is almost as straight up as the +One seat so hopeless for a child to nap. Their head lolls forwards alarmingly if they nod off.
Plus the Rumble seat has a maximum load of something like 15.9kgs which isn't much scope between a toddler not napping and them then growing out of it. Frigg and I both have hit the issue that our toddlers were 16kgs before they were ready to not use it, but both our little ones still nap. I tried putting the eldest in the main seat which was fine until the baby started to nod off but it wasn't nice to watch and I moved so fast to switch them round. But your children are lighter at present than ours.
But apart from that it's absolutely fab and if your son doesn't nap I woud be highly recommending it. There's some fab photos here under the middle green button of Friggs children interacting. Gorgeous to see. it's easy to push but long, easy to get up astep into a shop etc. A great idea but needs a recline and higher weight limit!