It’s a fascinating question, and I don’t know that there is an existing word.
Partly, that’s sexism, and partly, that’s the bias of historical linguistics in explaining derivation: Martina is the “feminine variant” or “feminisation” of Martin, and it doesn’t occur to people to describe the relationship of Martin back to Martina. In the rare instances where a masculine name is derived from a feminine (Catarino < Caterina is the only one that occurs to me), I still think noone has bothered to describe the pair Caterina, Catarino as anything.
Zeibura S. Kathau, I miss having the kinds of pub conversations you’re having.
Heterophylonym “other gender” is the pedantic answer, but it’s too long. Heteronym is already taken; how about phylonym “gender name”? (Phylon is both “tribe”, hence phylum, and “gender”.) There seems to be only very little usage of phylonym in the sense of “phylum”.
Genos has a similar ambiguity between “generation” and “gender”, and to my surprise genonym is already defined to mean a generational name: An Alphabetical Guide to the Language of Name Studies.