My method to animating a 2nd face can be the identical as that to animating a 3d face: By doing it within the 3d modeling program. I’d break up the face into a number of polygons after which animate their vertices. This could deform the face texture. For instance, flip a smiling face right into a frowning face or rotate the eyebrows from offended to unhappy.
One other method may very well be to make use of the 2nd animation system and attempt to render the outcomes to a render texture which is then utilized to the 3d mannequin.
By the way in which: TT Video games, the corporate that made many of the latest Lego titles, developed an personal inhouse engine for these video games. In order that they in all probability have their very own specialised tooling for doing these attribute 2nd facial animations on 3d characters.