I would look into finding a non-vented cap for the lower tank and then use the top tank to feed the lower tank through the vent tube. Can you give us an idea which tanks your using? Having a large vent tube to feed between the two tanks is important or else you will find filling the both of them takes a long time (have to wait for the lower tank to catch up if your below half full). Safety lock-wiring the lower tank cap on is a good idea just in case.
The trick is finding a non-vented cap to fit your lower tank.