...I love the scenario of one dom being kinder to the male slave, perhaps even to the point that the other dom is jealous of the male...
I envision a scenario of dominant lesbian (occasionally bi) with a female partner who is submissive to women but dominant (even hateful) toward men. The dynamics would have the female submissive tormenting the male as would a bratty sister. He, being sexually attracted to both, would be susceptible to her incessant teasing and would both dread and crave her attention.