So this is a rule that I am workshopping for a fantasy campaign. Several of my players are spellcasters, and they wanted to have that feeling of a coven that works together to cast a spell more powerful than any member could cast alone. This is what I came up with so far. I would really appreciate thoughts you guys have to make achieve this feel, but also balance it a little bit.
Characters with extraordinary attributes and access to the same boon may all roll to invoke the boon at the same level at the same time. The invocation time of the boon is increased by one time category if invoked in this manner. If the majority of the players passes the roll (i.e. ⅔ or ⅗), the boon is invoked at one power level higher than the power level rolled. Every player must maintain the boon. If one player stops maintaining the boon, its effects stops for all players involved.