I think it might be on the level as submitting to two Dominants, a husband and wife, for example. You have to care equally for those two beings who nurture your submissive tendencies. A single Dom/me may have his/her hands full with two subbies though.

If you supposed to a Dom and a Domme, you would have to trust these two implicitly and not one more than the other. The love would be shared in a triangular fashion in this situation as well. The same would be true if a single Dom had two submissives, thus making them siblings. I think trust and openness is the key in these relationships.