I think it's okay to mix itif it's done carefully &systematically, just like when handloading/reloading any other kind of ammunition.
If an "X" volume of grains of one powder is dropped, and then a "Y" amount of another powder is dropped, there really shouldn't be a problem as long as they areaccepted powders and the guidelines regarding volume equivalents are adhered too.
I've never heard of any incompatibility amoung different powders, and certainly in this case there shouldn't be any,especiallyif onlyusing 2 differentgranulations ofPyrodex.
Experimentation has helped to develop wildcat cartridgesthat havebecome acceptedfor commercial production, anda new desirable load just might be developed for an individual rifle that surpasses anyother loads that havebeen previously tried.
It always good to be cautious butthere'sno needto be afraid toexperiment within reasonable limits of acceptability.