I agree with some that I would assume there would be fights about he's kicking me, he's taking too much room, etc. Can 1 sleep on the couch and they can take turns or something?
It does depend on what they are comfortable with. I have slept on a twin with my son, but he and I don't fight too much and I as Mommy love the excuse to cuddle a little bit. If you had tried to get me to do this w/a brother when I was a kid it would have been a MESS.
As others have said, pile some blankets and let them sleep on the floor. Whatever works for you guys good luck!!!