I might have mentioned it before - the two of them being combined into one body by accident/in hopes of curbing Monty.

Hmmm... what exactly do you have in mind?

Monty muscle up and Freddy get fat
Or the other way around

How about two guys in a band that end transformed in both of them?

After a pretty big accident at the Pizzaplex involving kids daring Monty to tackle Freddy into the river at gator golf it's decided that while the animatronic get repaired the company will have a couple experimental biomechanical versions sub in.

These biomechanical versions of the characters are people sealed inside rubber / plastic / fabric costumes of the characters with an exoskeleton to fill out the character's shape.

These "bio-cores" are edged and possibly hypno'd into staying in character at all times