They take turns sleeping on the top.
In October we bought the boys bunkbeds so they could start sharing a room. Quinn finally moved into the nursery and is sleeping in a crib - he loves it! We got a great deal on the bunkbeds and got the bedding at Target. One set of sheets has different colored stars and the other has different colored cars, trucks, boats, planes, trains, etc. They just love their new big boy beds. We shouldn't be surprised, but we have to go in their room almost every night to tell them to stop talking and playing and go to sleep. I guess that comes with the territory!