I've been shopping this morning to look at all of the main ones. My thoughts from what I saw:
Mountain duet - side by side and very narrow and lightweight which was perfect but I was concerned at how narrow the carry cot was. Could imagine a newborn couldn't stay in there very long without being a bit restricted on movement.
Mothercare genie - great price but seemed a bit flimsy and eldest would be shoved in the bottom which i didn't like.
Nanu (think it's another Mothercare make) nice and stable and a better price but extremely heavy.
Bugaboo donkey - loved the way you could widen or narrow the handle depending on whether you were using it for a single, single plus shopping basket or double. Carry cot good size and fairly lightweight and very easy to manouvere. It was just too wide for my pathway.
Icandy orange - without doubt the best option. Still not cheap but it gave the most options in terms of where you put the children (which is good if you're concerned about a heavy one overbalancing the pram which is what I'm worried about) plus it comes with a built in buggy board so no need to buy extras. Was very lightweight and easy to fold up and compact.
Joie air twin - very flimsy although good price and narrow but I would only be happy putting older children in it not a newborn.
Joie duet - again carry cot seemed a bit narrow but wasn't a bad price and very lightweight.
I'm afraid I couldn't comment on the silvercross or egg (although this was gorgeous!) as they were way out of budget (as were most of them to be fair!).
Best thing you can do is find a shop which has them all there for you to try out as I had favourites I found online but in real life they wouldn't be practical.