Hi Wendy,
I just wanted to add that my two also shared a standard cot until they were 5 months old with no problems. I don't think you need to spend out extra on a "twin cot" I hadn't even heard of these before to be honest!
The other thing is eventually they will go into their own cot which then means you have to buy 2 seperate cots anyway if that makes sense? Two cots will also allow you to be more flexible with the way you set your nuresry.
We have to re-arrange our nursery this weekend and Twin 2 can reach the light switch, and Twin 1 can now reach the curtains!! It's like a mini-disco in there in the mornings now!
Jo