Sometimes magical effects that establish mental control render each other irrelevant, such as spells that remove the subject’s ability to act. For example, a creature under the effect of a hold person spell cannot be compelled to move using a dominate spell, because the hold person effect prevents the
creature from moving.
Mental controls that don’t remove the target’s ability to act don’t usually interfere with each other. If a creature is under the mental control of two or more creatures, it tends to obey each to the best of its ability and to the extent of the control each effect allows. If the controlled creature receives conflicting orders simultaneously, the competing controllers must attempt opposed Charisma checks to determine which one the creature obeys.
. . .huh, I just had a weird thought. Could a character with some form of mind-altering power use it on *themselves*, with a command of "Refuse to obey any order that comes from another person", and then voluntarily fail the save against their own spell? That way, if someone comes along and tries to mind control them, not only do they have to face a will save, but they would also have to beat the preexisting "control" in an opposed charisma check.
This is a fun idea... Get a necrograft installed and you, or an ally, can cast command undead on yourself... The possibilities here are entertaining at the least. Definitely an unconventional save insurance policy though.