I had a similar issue using plain jane RCA cables instead of shielded coax when splitting a signal. The subs would come on and stay on, and on, and on..... Had to unplug and sort out.
The fix in my case was to upgrade the sub cables to 75ohm shielded coaxial terminated with better ends. I ended up buying monster shielded RCA cables from the surplus section in princess auto. 15$ or so.
When I divided an already low signal I ran into issues. In your case, if not the cables, I would try a higher output setting from the AVR and turn each sub output down manually. This way hopefully each sub amp gets the input signal needed to behave properly.
Hope this helps.