They'll get used to each other. I put them both in the same room when youngest was around 15 months and eldest was 3yrs 2 months. Did the same bath routine, story time, put them both in and switched the light off. The novelty made the eldest filll the youngest's cot with books and teddies and there was lots of jabbering and chattering. A few tears from my youngest too. However after a few days they just got used to it if the other one was in a more rowdy mood. They would just fall asleep and the rowdier one would soon get tired and fall asleep.
Id just go for it and not get stressed if one is about to fall asleep and the other wakes them. They'll get used to it. Leave them to it.