If you have the room to keep them separate, I would suggest that until they are both a bit older and in "big girl beds".
At their ages now, they go through different sleep cycles with development and will be disturbing one another and you. For example: my son is 10 1/2 months old, and while MOST of the time he sleeps 12-14 hrs over night without waking, there are nights (like in the past week) where he wakes up crying. His crib is in our room, so it definitely wakes both my husband and I, and we remain awake until he goes back to sleep. If you have the two girls in the same room, it means you will have both kids awake, both kids tired and cranky the next day, along with you being tired an cranky the next day...that doesn't make for a good time for anyone.
When they are a bit older, they will probably really enjoy sharing a room. I nanny for a family that has 3 yr old and 5 yr old girls. Their girls share a room, and their beds are even pushed together. They love it most of the time, but there are times when one wakes up early or fights going to sleep and disturbs the other. But there are just as many times when the girls will lie together and read books or play quietly early in the mornings.