Don't automatically rule out side by sides. They are often ok to go through doors, but with the age gap I think a tandem would be better as could be used longer. Would you consider buying a different single that fits a buggy board, as even though 3yo is lazy, she is capable of walking, and could be trusted to stand well on a board. If you get one second hand, you could use it as long as needed, then revert to one of your other prams if you so choose.

We have a buggy board on our Quinny Buzz, the Lascal Maxi. It fitted with no problems and works really well for my just 3 year old.

We also have a double/tandem as we only have a 20-month age gap, whether we use the double or the buggy board depends on where we are going and if it's just me or my both my husband and me.

I would recommend that you buy a buggy board and a cheap tandem - there's one by Symths that people are talking about on here that sounds good for short-term use - or a second hand one. I wouldn't recommend a Phil & Teds as the older child is supposed to sit on top, so if they want to alternate being in the pram and walking, you then have to keep swopping the younger one from the bottom, and they're horrible for newborns because you have to use either the car seat or a swingy thing under the top seat that doesn't usually last until 6 months and the newborn is hidden but has no shade on their face if the sun is shinning. The Britax B-Dual is better because both seats are suitable from birth, the older one can be underneath and and it's cheaper but it's wide for a tandem.

I have just stuck a picture on my profile of the Smyths one if you want to have a look at it with kids in. It's quite heavy though, especially to lift, less so to push but I'm fairly tall tho so it could be different if you're smaller so have the handlebar angled downwards. IMO it seems like a good temporary solution, for those who will only need a double for a limited period of time. It's really really nice to have the baby be able to face you too, fairly rare in a tandem unless you're spending an awful lot more.
You do get what you pay for though, I would not be expecting it to last forever (even though it might, I'm erring on the side of caution at this point)

Do you mean the top photo? If so, wow! And I like the buggy seat thing. You have a nice set up.

Errrrmmm I had to check. I think it's the top photo... it appears like that to me anyway. The one with dd1 sitting in the lower seat and the top seat flat parent facing with dd2 in but you can only see her feet and a load of disorganised shopping in the basket? If so that is the Smyths Dimples Duo!

The buggyboard was very cool, I gave it away in the end because my two are 2y1week apart and DD1 really still needed a proper seat. We used it for 3 months though, quite happily for the school run and other very local things.

That sort of thing is actually not a bad solution possibly, for the OP although it won't fit either of her current pushchairs which is a bit of a shame. They are available on ebay, or they were a few weeks ago when I last looked.

Hmmm okay, parent facing, off road tyres.. The Smyths will be useless if you have quite a bit of "terrain" but the seat positions are great. To be honest we have too many cobbles, rubbish pavement and stuff here, it copes okay but air/terrain tyres and a sturdier frame does work better.
On the plus side I'm sure you can put a buggyboard on a buzz.. have seen it done.

I'm struggling now... Jane Twone? V light, claims to be 7.5kg (though I'll believe it when I weigh it myself!). Tiggy says push is also very light. Not very terrain-y though. Doubles are heavy with kids in, there's no getting around it. Maybe a narrow AT side by side like a MB duet or a new narrow Nipper would suit you?

Have a look at a Mountain Buggy +One. It should see you through the first 6 months easily by which time DD1 will be heading for 4 and hopefully things will be more positive to either move her onto a buggy board, or her walk, or if you dont mind the compromises with the baby then you still have the double option or a single that very quickly converts to a double without needing extra seats etc.

With your eldest already over 3, there are going to be very few options which don't weigh a lot, or have issues like pulling to one side (I would rule the Duet out for this reason), or which your eldest can "paddle" along the floor. I have an over 3 year old and we have a +One, a Peach Blossom 2 and have a Jane Twone on order but I have reservations about it - its easy to push fully loaded though and that alone is a big selling point with an over 3 year old. You really can't afford to be fussy!! The B-Dual may well work but you need to try it out. I really don't like the seating position for an older child (in either seat).

P&Ts haven't really worked for us - when the baby is newborn the eldest needs to sit on the doubles kit on top (not underneath) and headroom may be an issue on some models. But beyond that, the upper seat can't be reclined if a child is underneath and only some models have reclining lower seats. So you would need a reclining lower seat for the baby. But when the eldest hops out to go to the toilet or on the swings or whatever, then you have a weight distribution issue and as the baby gets older, you would have to either hang onto the pushchair - not possible if you need to help the eldest on the toilet or swings - or move the baby to the front to stop the pushchair tipping.

One other pushchair to keep your eye on is the Micralite Twofold. It is meant to be out this autumn but I haven't had an update lately. But that could well suit you. Or think about a Joovy Ultralight Caboose. It would mean compromises for the baby for the first 6 months ie a car seat but it works well for an older child. Other more off the wall suggestions which worked well for us are an ABC Adventure buggy (hopeless for the car but great to walk with) and the very long but easy to push Kolcraft Contours. The Uppababy Vista with Rumble seat (with a wedge for the baby in the seat) may be worth a look if your eldest is a light weight. The max weight is 15.9 kgs.

I have a P&T and find it pretty good. For the first six months, when your baby is lying flat, they are actually looking up at your face (albeit upside down IYSWIM) so you actually have a brilliant view of them, by the way.

Don't have a problem with weight distribution in the P&T either (doesn't tip when toddler gets out of front) - but maybe that's because I've always got so much rubbish weighing down the shopping basket.

And I do recline the top seat sometimes even with the baby in the back seat, doesn't seem to cause a problem. Having said that, I don't usually bother to recline for naps - my kids seem to prefer napping sitting up, and get annoyed if they wake up flat in the buggy.

My almost-3-year-old quite likes climbing into the back seat, actually, and doesn't mind if I put my 1-year-old in the front, so if he's going to be hopping in and out, then I put them that way round.

The other one I always like the look of is the Nipper Out and About. Not parent-facing, but think it looks very cosy with the two children nestled in together.

A side-by-side will be really heavy. I have a Maclaren side-by-side too, but far heavier/more awkward to push with the unequal weight distribution (and yours will be more unequal, so worse.)

I would go the buggy board route if possible I did that a lot with my first two, and it was great. The P&T better now though, as DS1 can also hitch a ride on the footplate if necessary!