Ok, the 2 gauges,
The right gauge is air supply, It there more for reference to make sure you have over 100psi plus psi to gauge. The right gauge is not needed.
The left gauge is zeroed (using the pressure valve) to 100psi before you hook up to cylinder using the quick disconnect.
The right gauge could be mounted between the regulator valve and the damper valve like some gauges are made. And adjust both gauges to 100psi.