If the main tank valve and auxiliary tank valves are both open, propane is drawn from both. The more full tank has a greater pressure, so feeds a a greater share of the total until the pressures equalize. After that, they will be roughly equal in share.
Typically, though, the valve for the main tank is shut off and the feed is from the aux tank only. That preserves the main tank as a reserve, which you can use if you run out of the other, or if you travel and leave the aux tank behind.