My Synodontis and my poly share the same cave in my tank - I also have a 2 inch rapheal catfish that sometimes goes in there with them.
some say its cause there's not enough hiding places and they just end up hiding behind each other.. or they might actually enjoy it.
I second this. If given places to hide they would and you would see them get more aggressive with each other. Its been my experience when given places to hide with driftwood they get territorial protecting there spots.