Dave,
You might try measuring the mag drop using a device like the Proptach
Digital Optical Tachometer.
www.proptach.com
Mechanical Tachometers do not have sufficient resolution to measure a small
RPM drop.
The Proptach has a resolution of 1 RPM and an update rate of 1 second and no
hook up required.
There should always be a drop on each mag if the engine stops in the OFF
position of the switch.
Another possible failure mode could be a short between the Right and Left P
leads but not to ground.
That failure could be in the wiring harness or the switch and would be like
having only one mag system.
Art Sundeen