I haven't really had that issue. My mix usually contains 3 different kinds of high quality cat kibble (my max was 5 types at one point). Every so often, one of the three will fall out of favor and I'll replace it with something else. I try to aim for similar shapes/sizes and nutritional content in terms of fat/protein/calories so that I don't have "healthy, healthy, and junk food."
None of the vets I've seen have said to stay away from a mix.
I'd say - take a look at the ingredients & nutritional content of the two you have right now... what the first 5 ingredients are (in order), the fat & protein %'s, and the shape, size & consistency/hardness. It's quite likely that if there's a substantial difference between the two, that might contribute to one being favored over the other.
Satin, for example, loved things shaped like an X and hated things shaped like a puck. In contrast, Bella hates X's and will eat pucks.