Multiple Mental Control Effects: Sometimes magical effects that establish mental control render each other irrelevant, such as a spell that removes the subjects ability to act. Mental controls that don’t remove the recipient’s ability to act usually do not 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 make opposed Charisma checks to determine which one the creature obeys.
That's what I thought of as soon as I saw the thread titleHenrix said:Yes, just as you can be friends with more than one person, you can be charmed by more than one. ;-)

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.