If a pellet/powder combo was experimented with, of course they would have to be the same brand with the same burn rate.
Different powders can be safely mixed & matched to make duplex loads even if it's just for experimentation purposes.
Flintlock shooters do it all of the time and it's even recommended by Hodgdon.
Flintlock: To insure proper ignition in flintlock systems, 5 grains of FFFFG priming powder should be placed into the bore prior to loading the main charge of Triple Seven or Pyrodex. Consult the loading data in this brochure to determine the proper charge for the caliber firearm used and the chosen projectile. The main powder charge should be reduced by 5 grains to compensate for the addition of the priming powder. While holding the firearm vertically, slowly pour the measured charge of Triple Seven or Pyrodex into the barrel. Seat the projectile firmly against the powder. Make certain that there is no airspace between the powder and the projectile. *See
WARNING below.
http://www.hodgdon.com/loading.html
Similarly, it's possible that there's a benefit from the different burning rates and characteristics of each kind of powder used in any duplex load.
And if people didn't safely experiment then many current ammunition rounds that were once wildcats would have never been developed.
I don't see anything inherently unsafe about duplex loads and I've been waiting to hear about duplex loads using Blackhorn 209 in sidelocks or inlines that utilize #11's or musket caps.
One would think that using a partial load of BH 209 would be cleaner than using a full load of a dirtier powder if it could be ignited reliably.
