from what i have researched in schooling twins, they recommend separation if one child is dominant or speaks or does things for the other. Otherwise they leave it up to you.
I have had friends separate their twins to give them some space, as it was just too much seeing each other all day and night, whereas others have kept them together.

personally from a teachers point of view, i did not have any hassles teaching twins in class.
There is plenty of research done etc. on this topic. esp from the net or from AMBA. We have info nights every couple of years about this topic in our club cause it is such a toughie for parents
hope this helps
odette