There will be no pressure difference between the two tanks as they are connected above the fuel by a pressure equalization tube, and the entire system is vented to atmosphere through the carbon canister. So no pressure buildup occurs.
The huge swings in fuel gauge readings are more a function of the it's design: the sensor float slides on two fine wires (and can mechanically hang up) and the wires are prone to fuel residue accumulation (which causes signal variation as the wires (with the float) provide a sliding resistive signal). With recently cleaned fine wires and frequent use the fuel gauge is reasonably accurate. Let the machine sit for a few weeks at a time and accuracy becomes hit or (mostly) miss.
