BoBoiBoy x Ben 10 Crossover 

While attempting to escape the null Void, Ben Tennyson ends up on Rintis Island where he meets BoBoiBoy and the gang. But considering they're already fighting an alien named Adu Du, BoBoiBoy didn't know whether Ben was an ally or an enemy.

Rules:

Takes place during BoBoiBoy Season 1 and the episode "Back with a Vengeance" from Ben 10 original series