Play along with a hypothetical situation, if you will.
If you happened to strike up a friendship with the Godfather of the Gay Cataloging Mafia who also happens to be a God (Cataloging) of SACO, and if he were willing to work with you to pitch butch and femme subject headings to the Library of Congress, what would you want those headings to be?
The headings would want to be separate, right? Something like
Butch (Gender expression) and
Femme (Gender expression)?
Should they be plural?
Should the parenthetical be Gender identity?
Something else entirely?