I really like your idea! I think it has a lot of potential as a rule.
One thing that might need some extra thought is whether this kind of coordinated attack should be allowed against any enemy. With so many dice rolled, the players might end up defeating almost any foe too easily, which could take away from the tension and danger of encounters.
My original intent was for certain creatures, like the Nazgûl, to be so terrifyingly powerful that players would want to avoid them at all costs .
Maybe this rule could be limited to specific situations or be used as an idea for the party to avoid a close encounter or some great danger. But overall, it’s a really solid concept and I appreciate you sharing it!I would love to hear about your adventures in Ring Goons!