The Voltwatch will do that if it's set up properly. And, in a two battery situation, as the original poster is using, the Voltwatch will give you the average voltage while in-use, which is exactly what a loaded voltmeter will do.

You don't need a diode anywhere. The setup as the OP is using is exactly what I am using on three of my planes and is just fine. In fact, I developed a battery problem in flight where one of the batteries developed a bad cell. I didn't realize anything in flight, then when I landed, I did my usual voltage check (I check the Voltwatch before and after each flight) and when I checked the one battery, it showed RED on the voltwatch (6 volt pack) and the other showed green. When I had them both switched on, the voltwatch was still in the green showing that the good battery was doing exactly what it was supposed to do.

The setup is simple: one battery per switch then each one is plugged into either the battery channel or a vacant channel on the RX (or use a Y connector cable and connect it to a used channel). The check process is also simple. Switch one on, switch two off, check the voltwatch while moving all sticks to maximum, should remain well into the green. Switch one off, switch two on, check the voltwatch while moving all sticks to maximum, should remain well in the green. Both switches on, then check the voltwatch while moving all sticks to maximum, should remain well into the green. Start the engine, the move all sticks to maximum while watching the voltwatch.. should remain in the green. Go fly.

CGr.