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 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.