To make your starter the night before:
Combine warm water with sugar, stirring until sugar is almost dissolved. Add yeast, stirring until incorporated.
Loosely cover bowl with plastic wrap. Set aside in a warm place for 10 minutes, or until foamy.
In a very large mixing bowl, combine flours, tapioca starch, and salt, whisking until well combined. Add warm milk and melted butter, once again stirring until combined.
Add water/yeast mixture, once again whisking until fully combined.
Loosely cover bowl with plastic wrap, leave out on the counter overnight - at least 8 hours.
When ready to use, heat up your waffle iron.
Whisk eggs together with baking soda, then whisk that mixture into your "starter".
Follow your waffle iron instructions to cook the waffles. Serve hot!