I had the same problem with a truck a while back. Turned out that a hole was rusted in the fuel line at the top of the tank, just where ut comes out of the tank. It was sucking air. Since it was on top, no gas was leaking on the ground.
I can't see it happening with 2 tanks though.....would be a real coincedence. There may be a hole in the line after the fuel line switch......While you are at it, check out the fuel line switch.