Ok. I've had a week to open and close the skimmers. What I found;

1. When I closed both skimmers to use only the main drain the pressure dropped by half, from the normal 18 to 9. The returns began blowing a lot of air bubbles.
2. Thinking that low pressure and a lot of air was a bad idea, I opened the skimmer farthest from the pump. Pressure returned to normal and the air bubbles stopped.
3. The next day I found I had still lost more water than was due to evaporation. (bucket observation) I then closed the near skimmer and opened the far one. The pressure stayed but I again got air bubbles, but not as bad as with both skimmers closed.

We had storms last night that added water to the pool, but the water level did not change. Based on the amount of rain we got, I'm guessing I lost about 1/4". I'm going to see what happens again today.