my identical girls are nearly 14 and they have moved their single beds next to each other (and have tied the legs of beds together so they don't move) so its now like they have a double bed as they don't like being apart. when they were younger we had bunk beds for them but i'd come into them mornings and 99% of the time they'd be in the same bed.
Sounds fine to me, my ID girls slept in the same cot to about nine months, they are now next to each other in separate cots. When they go into beds, I see them sharing again as the room they are in fits a double bed nicely but not two singles because of doors, cupboards etc. Obviously you will make sure they always sleep in the bottom of a bunk bed; I think you can get bunk beds with a 4'6" bottom bunk like compare.ebay.co.uk/like/330834282821?var=lv<yp=AllFixedPriceItemTypes&var=sbar&adtype=pla&crdt=0 this which would give them a bit more room as they get bigger.
My 14mo ID boys have shared cot since birth no probs. Sometimes they wake each other but usually go back to sleep if given toy and drink of water. I met someone whose grown up ID twins had voluntarily shared until age 11 (she had 2 beds but they chose to sleep in 1). The twins are going to have to soon move in with ds1 (5yrs) and it would be convenient if they could continue to share as the room is not huge and just having a bunk bed will give them all more play space.