Idea: Overlapping Characters

I just had an idea, it probably must've been thought about or actually done already, but I don't see anyone talking about it.
Its similar to Multiboxing, but the games are played simultaneously.
You move both characters at the same time.

Basically we would place 2 characters which matching speed stat (and preferably similar ranges) on the same spot: this way we'd be dealing twice as damage and "rolling" twice for loot.
With autoloot it'd be down to aiming and dodging.
Playing as Warrior+Paladin and boosting both specials at the same time would turn a few bosses into dust.
Maybe even different skill buttons so we could play i.e. Wizard+Priest and heal yourself.

We have one flaw already though. If you would get hit by a slowing projectile, unless it "hits multiple targets" it'll slow only one character.

Anyway, is this possible?