User blog comment:Rafe Adler/"Exodus" Discussion/@comment-79.115.77.118-20180512115514/@comment-1031724-20180515173014

Exactly!

It's like the ending of Sailor Moon R. Wiseman and his Dark Crystal were more powerful than one Silver Crystal, but two Silver Crystals - one from the present and one from the future - were stronger than him, and killed him.

Or Dragonball Z - Vegeta kills both Zarbon and Dodoria pretty easily while fighting them individually (though it took two tries to beat Zarbon once he transformed), but Frieza said if they joined their powers together they could beat him, which happens in the What-If story "Beautiful Treachery" in one of the games for PS2.