About what time did this happen? I wanna say I vaguely remember this. Sounds amazing.
There's a version of it apparently in Doom Eternal. You can have two players drop as demons on a doomslayer playing normally (if he's checked the box allowing it).It was one of the best ideas that never went anywhere. While the propensity for misuse was high they really should have brought it back for special events. With a few tweeks it could have really been something. Incorporated in to an expansion with a few high risk high reward zones in which players could run some of the monsters and there could have been some real cool emergent game play.