they merge if the vram of both cards are equivalent from memory. not 100% as diffrent users get diffrent results.
This is from an old (2015) article, can't find anything new.
they all merge differently between generations, running newer style cards net better results from the ones I know who run it successfully. Same brand gdrrr memory helps from 1 I know with this set up memory balanced pretty good running 1 2G & 1 4G cards. Had Samsung memory with latest updates got 6 gig. Pick your cards the right way helps all diffrent people experience diffrent results. If in doubt fire it up on crossfire & pull a gpu'z, load some drivers & see nothing to loose. Same gen cards might net you some ram if your lucky you may gain the more equivalent the better.