My girls shared most of their life and I shared with my sister my entire life and it was a double bed for us till we were teens.

Think about their temperments before you do it. Also realize that when they misbehave sending them to their room won't be an option - nothing like sending two feuding people to a small space.

My girls were opposite in the sense of one needed quiet and dark, one liked to listen to music, etc. We did have to set up a separate private space the one girl could go when she needed to. The younger they are when you do it the more they will grow to think it is normal.

Tell them that you will reconsider down the road and when they are older they might appreciate separate spaces or be such good friends it is like sleepovers all the time. How close they are in age might also make a difference.