As far as I understand following the line from the cylinder to the mask, you have one pressure gauge (indicator on starboard side of outside fuselage...the green thing, verifies positive pressure), then the shut-off-valve, then the pressure gauge (to the cockpit indicator), then the pressure reducer, then the mask with regulator.
When checking the regulator you also select emergency flow, and hold the test/reset button for 5 sec, to check that the pressure doesn't drop more than max 100 psig to verify that the valve is not partly or fully shut. If it is you will simply empty whatever pressure you might have in the line between the valve and the mask. Just my two pennies worth.