From the horse's mouth
Had several partial 1 lb. and 8 lb. containers of Accurate 2230.
Asked several reloading buddies about mixing them together so that for the next 1200 rds., or so, loaded would all be the same.
Their immediate answer "NEVER mix powders".
I e-mailed and called Accurate powder. After going thru several subordinates - hooked up with the Ballistical Manager - his answer --
As long as all the powder is the same (ie Accurate 2230), and purchased within 5 years of each other, mix and load.
DO NOT mix different powders.
I mixd, loaded, and took samples to range.
No difference in accuracy or velocity between mixed and unmixed.
If you have these kind of questions, call or e-mail the maker. I find their information to be much more enlightening and factual.
And remember, there is no such thing as a stupid question except for the one that is not asked.
Old School (which I am part of at 66) is great, but learning something new from the people who really know is invaluable.