Combine the flours and yeast in a ceramic or glass bowl.
Add the warm water and mix into a fairly thin, smooth batter.
Let the mixture sit for three full days at room temperature.
Stir the mixture once a day.
It will bubble and rise.
When you are ready to make the injera, add the baking soda and salt and let the batter sit for 10-15 minutes.
Heat a small, nonstick 9-inch skillet.
When a drop of water bounces on the pan's surface, take about 1/3 cup of the batter and pour it in the skillet quickly, all at once.
Swirl the pan so that the entire bottom is evenly coated, then return to heat.
The injera is cooked only on one side and the bottom should not brown.
When the moisture has evaporated and lots of "eyes" appear on the surface, remove the injera.

Let each injera cool and then stack them as you go along.
If the first injera is undercooked, try using less of the mixture, perhaps 1/4 cup, and maybe cook it a bit longer.
Be sure not to overcook it.
Injera should be soft and pliable so that it can be rolled or folded, like a crepe.