I have 2 girls - a 22 month old and an 18 month old. They've both shared a room for the last 10 months or so and it's been absolutely fine (touch wood). As sezwedz says, there's the occasional bit of mucking about and giggling but that makes me smile and realise they're normal and on the whole they both sleep right through. They ARE both still in cots although when I go and stay with a friend sometimes, they sleep in the same room there - the eldest in a double bed and the youngest in a travel cot. They sleep right through there too and actually, if the youngest wakes first and starts getting grotty, the eldest has got out of bed, gone to the cot and played through it with her until I've come in the room.
I found that having a night light was a HUGE distraction for them - meaning they oculd see each other so were stimulated, rather than winding down and sleeping. So if your children are going to share a room I'd recommend complete darkness for them. Oh, I also put them to bed at the same time, with the lights on, they both know each other are there, I switch the light off and leave the room (after a story or twenty).
Good luck!