A gas supply system for providing high pressure (HP) gas to a low pressure (LP) gas destination, having a primary HP gas unit and a reserve HP gas unit, which provide regulated lower-pressure gas to a supply manifold, and an LP destination regulator that provides an LP regulated gas supply to a consumption subsystem. A one-way flow valve in fluid communication from the primary HP gas unit to the reserve HP gas unit, ensures that the reserve HP gas unit remains substantially full, even after numerous cycles of depletion and replacement of the primary HP gas unit, during which the HP supply is provided by the reserve HP gas unit, which helps to avoid the risk that the reserve tank pressure and supply might mistakenly, unexpectedly or unintentionally be depleted.