I'm in the one powder at a time on the bench.
The only time I pour powder into a bottle it didn't come out of is when I get to the bottom of a bottle, and it's not enough to load more than a few it gets dumped into the bottle of the same powder I open. I then mix it well.
I've not mixed two powders other than this.
If I did, even as limited as my budget is I would use it as fertilizer, but then again I only buy powder in 1 lb. bottles.
Leo