I have the firstwheels city twin and I love it 
It has carrycots, seats and with car adaptors can take the maxicosi seats. Each type of seat just slits on and off, and you can have different types on each side and each side facing either way independently.
It is light, though the footprint of the chassis is quite large when folded.
It is side by side, which was important to me as I didn't want one twin stuck behind the other if I could avoid it.
It looks fab
I kind of feel like I should have a nanny uniform on and be pushing it round Chelsea, not taking it off road over to the farm shop! It copes with the off road stuff well though (though perhaps not as well as one with large front weeks would).
It is narrower than a standard disabled access door, so I can plan days out by asking about disabled facilities 
I really love it. I'm not sure many people stock it (though I've no idea why!), but it is available online. I got mine from the kiddicare store, but it was the stock model as they were discontinuing to make room for out and about nippers. They do come up regularly on eBay though.
Good luck picking one - I would advise really taking your time. It is one of te most expensive items you need to buy, and I think it is the one that will affect your life the most. If you choose a cot and then regret your choice, it will be far easier to live with than referring your pushchair choice. So take your time, borrow things to try if you can (TBH I'd take an empty borrowed pushchair round town for an hour if you can - will help you see what is important to you!)
Good luck and enjoy your twins
It really is a fabulous adventure every day 