What is it called when two senses of the same word are etymologically independent?

I believe homograph is what you're looking for.

Homograph:

Homographs are words that are spelled alike, but have different meanings and sometimes different pronunciations. The root graph comes from the Greek word meaning “drawn or written,” thus these terms are written the same. For example, stalk is both a plant stem and a verb meaning to pursue stealthily. Homographs also have different etymologies.

Homonyms are words spelled or pronounced alike but different in meaning. Since homonym is used to (ambiguously) describe either a homograph or homophone, it can cause confusion, though it is often heard in classrooms in early grades. The root -nym simply means “word” or “name,” so it applies more broadly than its counterparts.

[Dictionary.com]