Hello, I have materials stored in separate USD files that I am referencing. How can I mix these materials?

AFAIK, that's not a feature of USD

The edit material network lop node got beefed up and seems very stable now, you can try to load both referenced materials there and mix them onto a third or just copy the contents onto a new material library.

I had already tried that. Unfortunately, without success. I had hoped to achieve this via "Import/Specialize USD Material," but that didn't work either. Thanks for the help anyway.