My kids shared a room from when DD was about 9 moths and DS was nearly 3.5. She was just sleeping through when we moved her, it worked rally well and there were very, very few occasions when they woke each other at night. One shouting for us, one with a sickness bug during the night never woke the other.
We have just split them into spirit rooms, DD slept alone for the first time in her life a week ago and she didn't like it! She has adapted really quickly though and loves her new room.
I would put them together, it really seems a more natural choice than a toddler sharing with parents - you need your space too.
Laura