Mixing powders can be done with great success if you have a pressure gun, proper, scales, correct mixing equipment, and the powders in question lend themselves to being mixed. Extruded and ball would be out of the question, in short, unless you are a well equipped powder mfg, or in some cases powder importer bringing in tons of powder, and have all the right gear, and a way to test and retest the final product, do not try it. Duplex loads were popular many years ago, back in the day the burn rate chart had 25 or so powders, there is far to much danger for the potential gain. I remember when the 454C shooters were dipping into the duplex and triplex loads, it was stupid then, but really stupid now. The firing of more than one burn rate isn't the only problem, but basic type, a double based powder, due to its nitro content, can and does act as an explosive rather than a flammable solid under the right circumstances, duplex and triplex loads create those type circumstances. You friend may in fact have a book that talks about mixing powder in order to achieve a new burn rate, but the chances are that book went out of print (except as a historical ref) 75 years ago. The answer given by diverdon is exactly correct-good advice.