What happens during automatic film processing if you put 2 films in?

Answer 1

More likely than not, the film would fail to develop properly, or jam the machine. If the machine didn't jam, you would end up with images from the two rolls of film superimposed on the prints.

If you are getting prints like that, however, the greater likelihood is that you did multiple exposures in the camera. That could come from inadvertently depressing the multiple exposure button while winding, or by putting an exposed roll back into the camera by mistake.