One line, the one going to the carb, should have a "tee" in it, with a fuel line running off the "tee" to act ast the fill. The fill line is simply plugged for flight. Correctly installed that fill line will pass through a "fuel dot" to permit sliding in and out of the fuselage for fill access. The other line acts as the vent and it never gets plugged unless the plane is in storage. Plug it then to limit odors or leakage.

Inverted flight is not really a big leak issue. If you flew all day inverted I doubt you would see more than a minute or so difference in flight time between upright and inverted.