Is there a name for this relation on CFGs?
This is a kind of (surjective) homomorphism,  but the traditional definition is defined only over terminal symbols, not nonterminals. 

Your definition would merge unrelated nonterminals from the first grammar with each other in the second grammar, thereby allowing (the image of) unrelated parts of the grammar to get mixed up with each other. This probably does not have nice properties, at least compared to the traditional definition of homomorphism.

That said, I can imagine that it is useful for some kinds of modelling, of AOP, of dynamic binding, of extensible grammars. I'd be interested in knowing what your application is.