Place white rice flour, brown rice flour, tapioca starch, sugar, baking powder and salt in the bowl of a food processor. Run food processor for about one minute to combine. Add shortening. Pulse to combine, about five medium pulses. No large pieces of shortening should remain.
Store mix in an airtight container at room temperature for up to 8 weeks.
Make sure to combine until mixture is coarse crumbs.
This mix will stay fresh in an airtight container for up to 8 weeks.