The Final Fight

[Tami]

Name two characters who, if they fought each other, would cause massive amounts of collateral damage on the largest scale possible.

Edit: Caveat - must be two characters who, on their own, don't have enough power to cause massive, widespread damage.

[Beadle]

The One Above All and The Presence?

[BitVyper]

Staying sub-omnipotent, DBS has basically said that any confrontation between two gods of destruction will absolutely destroy the universe it happens in.

[Tami]

Making an edit in the OP to make it clearer.

[Postmania]

Mad Jim Jaspers and the Fury kind of did this

[The MunchKING]

Well it can't be a Final Fight without Mike Haggar! I'll go with his fight versus Galactus!

[Sharkerbob]

I think the scenario your asking for is two normally low(er)-level characters whose clashing powers would react in such a way as to spiral way out of control and cause significantly more destruction than normal as a side-effect?

Having a hard time thinking of examples where this could be the case without involving people who can already cause huge damage on their own, but I'm sure there's some kind of combination of abilities there. You might have to get more indirect/esoteric. You might be looking at something where one person's power accidentally turbo-charges the other's power, or two energy casters whose energy-clash causes some kind of chain reaction.

As more of a scenario type situation, a 28-Days Later "zombie" fighting Quicksilver or Multiple Man in the middle of New York, there's a high chance either of them get infected from a random drop of blood, and suddenly, Zombie Quicksilver has converted thousands of people in under a minute, or Zombie Multiple Man replicates himself as many times as possible and manages to convert thousands as his self-army surges through the city.

Or you might have, I dunno, one of Superman's wacky Silver Age villains shoot him in the face with Red Kryptonite Lasers, and Superman suddenly becomes 10 times more powerful, but thinks the Earth is a Tootsie Pop and started eating his way to and through the Earth's core.

Otherwise, I'm sure there's some Mages who could throw badly-reacting spells at each other and end up summoning Cthulhu, or two energy casters getting into a beam battle that causes a nuclear-grade explosion or something.

[Jackalope89]

Ranma's Hiryu Shoten-Ha can have incredibly destructive capabilities, but it depends on whom he's fighting. When he used it on Ryoga, it sent pig boy flying. When he used it to counter Saffron, an entire mountain was destroyed.

[T51R]

Shin Getter VS Gurren Lagann. The fight will probably evolve the the point it destroys the entire galaxy.