The Toxic Avenger IV: Citizen Toxie (2000)

When the notorious Diaper Mafia take hostage the Tromaville School for the Very Special, only the Toxic Avenger and his morbidly obese sidekick Lardass can save Tromaville. However, an explosion results in some unforeseeable consequences where it creates a dimensional tear between Tromaville and its dimensional mirror image Amortville.