That's a good question. For instance in flintlocks made to shoot pellets, they suggest a duplex load of loose Goex and then some pellets on top of that. The thought being the Goex when it flashes will ignite the pellets. Yet Hodgdons tells you not to mix powders. So technically you have went against the manufacturers recommendations.

Personally I would not mix loose and then pellets to "make a load." Shoot the pellets off and then shoot the loose powder. Most pelletloads, say for 80 grains is a 50 & 30 grain pellet. You can get close with the two different size pellets but I hate the cost factor there. When you mix the two, I think you will get no consistancy. But then I have never done it. Maybe there are those that have and had good results doing it.