If two power nullifiers...

Use their powers on each other what happens? Since the power to nullify is a power, and if they attempted to nullify each others, what would occur?

Nothing

There's only one character that I know of that does that well enough and that's Cyttorak. Keen readers would have already known that Cyttorak power and Oblivion's power already clashed once before, and Oblivion ability to erase and obliterate was negated by Cyttorak's own. There is also proof in the War of the Seven Sphere, Infinity Wars, Fear Itself, AvX where Cyttorak has negated powers before.

@Icarusflies:

They surrender.

@YourNeighborhoodComicGeek: All Star Superman reference. *Claps*

@The_Lunact_And_Manic said:

Nothing

The universe would explode.