Dd is 19mths and DS is 12 weeks and we have a 3 bedroom house although the third is used for DH's office as he runs his own business. At the moment it's fine but the bedrooms are small so if kids had to share it would make it hard unless had bunks and they are to young for that now. If rooms were bigger I'd have no problem with kids sharing. DS is currently in our room/bed but he has no where to store his clothes which I find frustrating as rooms are ending up cluttered. Will be another year before we start building a new house so until then will have to find a way for it to work I guess. I shared a room as a kid with two other siblings (my older sis always got her own room) but room was large so was no problem.
So things to take into consideration is room size and if will fit beds and cupboard room storage for kids clothes etc.