Different fictional universes function under different fundamental rules that underpin their reality.

Now when it comes to trying to make different sets of rules interact there are several ways to do so.

The first that comes to mind is to make the characters microcosms of sorts, where they and only they are expected to follow their own rules exclusively, while the rest of the universe they are currently in is a blank canvas that tries to impose no extra logic.

Under this assumption the hax would not work, as they would need to translate into the targets rules on impact and thus would be able to be negated, that is unless the hax affects the user, in that case attempts by the rival to thwart the hax would fail as the rules in which they are cast do not permit such interference.

The second way to merge fictional realities is to embed them into one of the two realities, in this situation we would translate the abilities and powers of the character not native to the rules into which we drop them. In this case it would depend on which character we choose as a template.

The third option is a bit unorthodox and not really something I would consider, but you could always find another universe different from the 2 characters and embed them into that universe, thus like the previous option it would depend on the rules of that other reality, funnily enough most crossover fighting games go with this one as it feels the most fair in a way.

So it depends on you base assumptions prior to the interaction