Bunk beds, big beds...little kids?

I've been reading up on all the older topics, but they didn't really answer my question.

DD1 is 3yr 2m, and DD2 is 9.5m - they are in separate rooms at the moment; we have a small 3bdroom house.
At some point we want them sharing a room, but will have to be in bunk beds as the rooms are not big enough for 2 single beds plus all the other paraphenalia kids have!

I wanted your thoughts and advice on this stuff....

How young is too young for DD2 to go into a big bed? (obviously over 1 yr old, but we didn't move DD1 until she was 2). What about ages for a top bunk? I would hate for DD1 to fall out...

the reason I'm wanting to know is because at the moment, my computer desk is in DD2s room, and both their cupboards and under their beds are bursting at the seams with stuff that has to be in the house...I wonder if I should move them both into the same room (one in a cot, one in a bed), and make the (then) spare room into a study/playroom with their toys, or put their chests of drawers in with the study stuff, or simply whether to wait until they can share a bunk bed so I don't have to try and cram them both into the same room.

What are your thoughts/ideas? how have those of you managed with 2 kids and a small house?
Looking over my post it seems like a pretty stupid question to be asking, I guess I'm just thinking out loud and wondering what others have done

Edited by MuppetsMama

My two are in bunk beds DS is 3 1/2 and DD is 19months. Works well in our house as their room is to small for 2 singles. We did put DD into DS's room in the cot at first though. And we just have a strict rule that the only time DS is allowed on the top bunk is when he is going to sleep.