Dragon Ball Z: Fusion Reborn(1995)

Dragon Ball Z: Fukkatsu no Fusion!! Goku to Vegeta

Not paying attention to his job, a young demon allows the evil cleansing machine to overflow and explode, turning the young demon into the infamous monster Janemba. Goku and Vegeta make solo attempts to defeat the monster, but realize their only option is fusion.

(Source: Anime News Network)