More minds gives more close combat damage? How does that work? Only thing I can think of is that you have lots of extra weapons for the extra brains involved, otherwise:
7.4: Taking Action wrote:A multi-brained or superintelligent Creature can even take two or more completely dissimilar Actions in the same turn [. . .]; however, it may not use the same weapon, hand, or equipment item for more than one Action during the turn.
I'd treat each extra head as one helping of Multitasking (plus one of Multidexterity, if the creature has enough arms or weapons to need it). If the head gets chopped off, the creature loses those associated advantages. (Unless it's the last remaining head, in which case it loses the advantage of being alive.)