don't think that your test is quite representative of what I was suggesting, since you are disabling one regulator but leaving its body in circuit. If their is any instability in the regulator it could be caused by the its structure, since the air flow path is convoluted. For example the water separator function is achieved by creating a vortex in the bowl.
this is true Richard. not the best example - I could unplug both ends and just use a coupler.
That said how accurate are the gauges we are using anyway?
Even if these devices were perfect we know that no two records are identical.
Always trust your own ears. Use measurement devices. PSI gauges, alignment devices, VTF scales to just get into the ball park. Fine tune with your ears.