Tiggy would be better at this, have you had a look at best buggy? They have lots of reviews on there.

I'm due dc2 in October and will have exactly 2yrs between mine. I've been hunting round for a while and have been back and forth so many times now! I'm by no means an expert but there are tonnes of threads for doubles, it really depends on what you class as important....

I've just ordered an iCandy peach blossom 2 which does as you describe, but there is quite a long delay on a new one and mine isn't due in until late September. Have a look on the threads here and best buggy (sorry can't manage links on my phone) to see if the peach blossom would do as you needed and try to pick up a second hand one to avoid the wait for a new one.

I also looked into the britax b-dual quite a lot... It's very similar to the peach blossom and although a bit heavier, the wait for one may not be as long (they may even be in stock

Ah, have just re-read your OP re small fold and small boot... Check te width of your hall v the BJCM twin as its quiet narrow for a twin iirc and fold completely flat.

Have you considered a sling/pram/pram/ buggy board combo? Wouldn't work for us as we have no car but it might solve the space issues if you drive a lot?! Plus baby is due in summer so weather for your oldest dc to walk/ buggy board is better than an autumn/winter baby (which we will have...)

First of all if you have a small boot, forget the Graco because its a huge chunk folded! I watched someone recently struggle to get on into their estate car boot and they ended up having to put it across the back seat!! I haven't had the Stadium Duo but the Quattro Tour Duo was definitely the heaviest double pushchair I have used - incredibly heavy to push and not great to lift either as its such a big chunk. I dont see the Stadium Duo being much different but I will stand corrected.

I think to be honest it will be hard to get a tandem where both children have a view ie not one underneath with a small boot, and needing a small fold. I think you will need to compromise if size is most important. The smallest fold tandem I know of is the Baby Jogger City Select but with an already two year old, I wouldn't be recommending it. It will very quickly become very heavy to use although it would be great for the first 6-12 months which may well get you by depending on where you walk. That would give you a choice of seating options.

I would however recommend looking at the iCandy Peach Blossom (1 or 2 with your age gap), Britax B-Dual, Jane Twone or Babystyle Oyster Max.