Interesting. I either shoot magnum slugs, or #4 magnum buckshot. I never thought about mixing the two, but I've also never loaded shotgun shells.
Now at my age, the 12 guage is loaded for protection only. #4 buck in a magnum load in a Model 1100 I built for fighting back in my younger days.
My long walk days are over.
But seriously, what is the purpose of mixing the two?