I have a gallon of BRS of each.
Can I mix 1 cup each of them 50/50 in a gallon of RODI for 2 part dosing?
I saw 85/15 soda to ash somewhere, would a 50/50 mix raise ph to much? That wouldn't bother me too much being winter and the place is closed up a bit.
You can mix the supplements in any ratio and the tank likely will be fine. Using about 15% dry soda ash to 85% dry baking soda gives a mixture that affects pH the least. Lots of people use 100% soda ash with no problems, though. Is there a a specific problem that your tank is having with pH? Some tanks are better off with baking soda.