I suspect the answer may be yes, since you can have multiple I_SOURCE and I_EFFECT, but can an interaction have more than one I_TARGET?
EDIT: Might as well throw an example out here.
[INTERACTION:GOOD_REGION]
[I_SOURCE:REGION]
[IS_FREQUENCY:100]
[IS_REGION:ANY_TERRAIN]
[IS_REGION:GOOD_ONLY]
[IS_REGION:SAVAGE_ALLOWED]
[I_TARGET:A:CREATURE]
[IT_LOCATION:CONTEXT_REGION]
[IT_AFFECTED_CLASS:NEHUNDEAD]
[I_EFFECT:ADD_SYNDROME]
[IE_TARGET:A]
[IE_IMMEDIATE]
[SYNDROME]
[SYN_NO_HOSPITAL]
[SYN_CLASS:MORR]
[CE_SPEED_CHANGE:SPEED_PERC:80:START:0]
[CE_PHYS_ATT_CHANGE:STRENGTH:70:0:TOUGHNESS:70:0:START:0]
[CE_SKILL_ROLL_ADJUST:PERC:70:PERC_ON:100:START:0]
[I_TARGET:B:CREATURE]
[IT_LOCATION:CONTEXT_REGION]
[IT_AFFECTED_CLASS:UNDEAD]
[IT_AFFECTED_CLASS:VAMPIRE]
[IT_REQUIRES:NOT_LIVING]
[I_EFFECT:ADD_SYNDROME]
[IE_TARGET:B]
[IE_IMMEDIATE]
[SYNDROME]
[SYN_NO_HOSPITAL]
[SYN_CLASS:MORR]
[CE_SPEED_CHANGE:SPEED_PERC:80:START:0]
[CE_PHYS_ATT_CHANGE:STRENGTH:70:0:TOUGHNESS:70:0:START:0]
[CE_SKILL_ROLL_ADJUST:PERC:70:PERC_ON:100:START:0]
[I_TARGET:C:CREATURE]
[IT_LOCATION:CONTEXT_REGION]
[IT_AFFECTED_CLASS:CHAOS]
[I_EFFECT:ADD_SYNDROME]
[IE_TARGET:C]
[IE_IMMEDIATE]
[SYNDROME]
[SYN_NO_HOSPITAL]
[SYN_CLASS:MORR]
[CE_SPEED_CHANGE:SPEED_PERC:80:START:0]
[CE_PHYS_ATT_CHANGE:STRENGTH:70:0:TOUGHNESS:70:0:START:0]
[CE_SKILL_ROLL_ADJUST:PERC:70:PERC_ON:100:START:0]
[I_TARGET:D:CREATURE]
[IT_LOCATION:CONTEXT_REGION]
[IT_AFFECTED_CLASS:GENERAL_POISON]
[I_EFFECT:ADD_SYNDROME]
[IE_TARGET:D]
[IE_IMMEDIATE]
[SYNDROME]
[SYN_NO_HOSPITAL]
[SYN_CLASS:MORR]
[CE_SKILL_ROLL_ADJUST:PERC:100:PERC_ON:100:START:0]
EDIT2: Never mind my question, it is possible. Don't know what would happen if I set all of them to have the same target id, but probably something buggy.
Instead, let me change my question: Can a region have more than one regional interaction?