Can two different species of fungi from two different genus' mutate into a new species? i just want to know if it's a possibility, and how gigantic or slim the chances are of it happening.

Yes, it is possible.

Explanation:

In this case, I'm guessing you mean breeding.

If that's the case, they will form hybrids.

Between two different species, inter specific hybrids will be formed.

Between two different genus, intergeneric hybrids will be formed.

There's even rare cases of breeding between two families.

So while it is possible for different species and/or genus to breed and form new species, in interspecific breeding for instance, they will have features similar to the parents but turn out sterile.

So to answer your question, it is possible.

not sure if this what   you'r looking for would be percent composition

the question isn't complete here so i can't right an answer '_'