Place white rice flour, brown rice flour, tapioca starch, sugar, baking powder, and salt in the bowl of a food processor. Pulse about 5 times to combine.
Add the shortening. Pulse 5-10 times to combine. No large pieces of shortening should remain.
Store mix in an airtight container at room temperature for up to 6 weeks.
Notes
Mixture makes 582 grams, which is about 4 ½ cups.
Make sure to combine until mixture resembles coarse flour.
This mix will stay fresh in an airtight container for up to 6 weeks.
A stand mixer can be used, but a food processor is recommended to ensure everything mixes completely.