My little dogs do things to each other that would make 95% of other dogs irritable. For example, Chulita was humping Jose` and he was wanting to walk across the room. He didn't react to her at all. He just dragged himself along with her hanging onto him like one of those women in the old time movies that hang onto a man's leg as he drags her along the floor, trying to leave and she's saying, "Oh pleeeeze, I love you, don't leave me." LOL. I finally told her to leave him alone, but he just goes with the flow about anything and everything. I've never seen him get pissy toward the other dogs for anything except if they walk near his Dremel and that's just a little, mild warning growl to make sure they know that it's his Dremel and since they couldn't care less about the Dremel, there is no problem. Most of the time, it is put away though. And Jose` will sometimes step on her when she's hidden under a blanket. She gives one little, short growl to let him know she's in there and he steps away and that is the end of that. They seem to have it all worked out between them. Toker is the sweetest natured girl too and none of them every have an issue no matter what the other one does.
So, when I leave, I'm pretty sure all they do is sleep on the couch and/or a chair because there's really nothing else going on. When I'm home, they don't play with each other or a toy very much, unless I'm sort of in on the act too. It's interesting, but they prefer it when I'm somehow involved. Chuli will get a toy but she keeps looking at me to see if I'm looking and if I'm not, she generally loses interest in the toy.
I wouldn't automatically leave just any dogs together if I weren't home. But it just happens coincidentally, I guess.... that I have never had any problem, any doubt... over the course of years with my particular multiple dogs.