Manufacturers will tell you not to mix powders... The reason is the inconsistency that the mix will produce might never be repeated. Also you could change the overall strenth of the powder that if used wrong could make for a somewhat dangerous load.

Personally I think you're better off shooting each brand as an individual powder. I am sure you could mix them and it would shoot off just fine. Its just that, lets say for instance you get an accurate load... can you repeat and mix the powder the same again? Is one powder with finer grain working its way to the bottom of the container as it moves about? All of this leads to inconsistency and that leads to poor accuracy.

The only time I mix powders is when I duplex a load with say.. Goex black powder and then a substitute powder so I get better ignition.