+1 on Marc. I transport mine behind the front seats with the handles touching the floor and the body snugged up against the wall. I then place bags or other cargo between them. The bells don't get a chance to move and smash anything. I have been doing this for years and routinely carry them without incident. Matter of fact, I have two 20's, two 24's, and a 28 in my car right now. My car does have separate "wells" behind the seats, thus KBs cannot roll from one side to the other.
With two you will definitely have to put something to keep them from rolling around. It can be a small block of wood, piece of carpet, anything bulky enough to take up the room and not really heavy, as you are simply preventing momentum. The important part is to ensure the handles are touching the floor and their bodies are snug against the walls as that "anchors" them from rolling in those directions.