How can I drain a two tank compressor?
You don't fully open the drain. If you do, the air pressure will create a vortex and stop it from draining. Crack it open until it spits and starts coughing up air, then cut it back some until all it's doing is gurgling.

Repeat until you can't get it to gurgle anymore.

Pipe it with some flexible copper to a nearby drain for simplicity.