Currently we have one bedroom for our two daughters. Until what age can they share their room?
Same-sex siblings don't need separate rooms, at least not if they're roughly the same age. Sharing a room teaches valuable social lessons. It's more a matter of how well they get along in general, and how much space you can afford.
I wouldn't put a 2-year-old in a room with a 12-year-old though, or two siblings that terrorize each other.
As one of 5 I shared a room with 1 brother and a sister for many years, then as elder siblings began to leave home we all started to get our own rooms:-)
My suggestion - make do with whatever space you have. If they fight and you can't solve that problem, then it may make sense to split them up, but otherwise it will depend on the space you have in your house/flat etc
There doesn't seem to be any law in the UK to stop children of the opposite sex sharing a room (advice on overcrowding) - which makes sense. I can't see why the same wouldn't be true for mainland Europe. So I think we can assume that children of the same sex can share a room as well.
With no legal reason compelling your daughters to sleep in separate rooms it becomes a personal choice. I can't say when they should stop sharing as that will be different for each set of siblings, but my 2 boys won't be separated unless I think it is affecting their sleep. Obviously as they get older the criteria may change.