That's right. Only one voltwatch. Two is just not needed as they both get connected to the same place, electrically, that is, so they would measure exactly the same thing.

One voltwatch, two batteries plugged into either two vacant slots or connected to the receiver via a Y connector, one switch per battery. Turn one on first, check the voltwatch, turn it off, then turn the second battery on, check the voltwatch, turn both on.. fly. Just as I said in my earlier post.

CGr.