You would use somthing like this:
Code:
void main() {
object oNPC1=GetObjectByTag("npc 1 here");
object oNPC2=GetObjectByTag("npc 2 here");
ChangeToStandardFaction(oNPC1, STANDARD_FACTION_PREY);
ChangeToStandardFaction(oNPC2, STANDARD_FACTION_PREDATOR);
}