For dd2 when we put her in the bottom bunk we just tucked one side in and had a rail on the other. She would wake and still does. She walks into our room, I just grab her hand and walk/carry her back to bed, kiss her good night and leave. There has only been a few times where they have woken each other up, so not too bad over the past 2 years! I have found both dds asleep on the floor at one stage or another.... actually we found them sleeping together in the same bed one night! I havent really got anything to say on tt as I just did it as I saw the need for it. Im in the process of tt dd2 and shes 2years 4months. I wasnt in a rush to get them on the loo.... you will know when she is ready or at least showing signs of it.
Good luck