I know this is necroposting and I'm sorry for this, but it's important to report bugs, even on old scripts, because people still use them.
Anyways, you have to know that your damage calculation works fine when it targets a single enemy (the stats of all the people in coop are included in the calculation, which I guess was intended) but when there is more than one target, then the damage on the other targets only takes into accounts the stats of the caster of the coop spell.
Example : A coop skill that has a damage formula of atk*1 (to be simple). Character 1 has 400 attack. Character 2 has 200 attack.
If the coop skill targets all the enemies, then enemy in index 1 will take 600 damage, but the other enemies will only take 400 damage (if I cast the skill with character 1) or 200 damage (if I cast the skill with character 2).
I know this script is old and you are focused on MV, but if you could fix this, that would be great. What is probably pretty simple for you is completely beyond me.