Rinse the moong dal and chana dal well, then soak together overnight in plenty of water.
Drain out all the water from the soaked lentils completely before blending.
Add the green chilies and garlic to the food processor jar. Add half of the drained lentils, close the lid, and blend to a smooth batter. Scrape the sides 2 to 3 times to bring any whole lentils back in. Press a bit of batter between your fingers; it should feel smooth with no coarse lentils.
Transfer the first batch of batter to a bowl. Process the remaining lentils the same way, then transfer that batter into the same bowl.
Add the turmeric, red chili powder, salt, and cilantro to the batter and mix well.
Lightly brush 2 large baking sheets with oil. Take a bit of batter in your fingers and drop small portions onto the trays. Wet your fingers to help the batter release easily, and keep the drops close together so they all fit on the 2 trays.
Set the trays out in direct sun and allow them to dry for 8 to 10 hours. A fan running nearby helps keep flies away, especially in summer.
Remove the sandge from the trays and separate any that are stuck together. Fully dried sandge makes a rattling noise when mixed or dropped on the tray.
You can also sun-dry the sandge for a second day before storing.
Transfer the fully dried sandge to an airtight container. They keep well in the pantry for up to 6 months.