Thanks for the replies, guys. But I have to say, I've never understood this part, even though I've heard it before:
Don't even mix different batches of the same powder.
There might be some variance, batch to batch.
Can't ever be too careful when messing with gunpowder.
If either batch of powder would be safe to use on its own, how could a thoroughly-blended mixture of the two batches be unsafe? Of course, I'd be working my loads up beginning from starting loads, but I do that anyway with any new batch of powder.
As an example, I have two 8-lb jugs of WC860. Rather than starting with one jug, working up an optimum load, and then having to do it all over again when I finish the first jug and open the second, why couldn't I just mix it all together from the start, work up a load, and then use that same load for the entire sixteen pounds?
EDIT - I just saw that several of you addressed my question before I was even done typing it. Thanks again!