I have two versions of a face mesh (via Shift+D--should be identical XYZ positions); one has correct eye (and nose) weight-painting and the other has correct lips (and chin) weight-painting. The only vertex group they both share is "head", since each object's specific region's weights were subtracted from the head. Is it possible to add/transfer one object's VGs (e.g. lips) to the other (e.g. eyes) and also correctly combine (subtract) the relevant weights from the overlapping VG (e.g. head)? I tried applying the commenter's answer here (TL;DR: Data Transfer modifier), but got strange midline-lips and lateral inner-mouth motion. (The inner mouth is not shown.) Thanks!
Face with correct eyes rig (JP names are "head" and "eyelash top/bottom 1, 2, etc."):
Lips (masked from identical face mesh) with correct lips rig (JP names are "head" and "(center)top/bottom lip L/R):