Yet another iteration, which maintains the separate test function and ability to control ship's power to each Pmag, using 3 toggle switches in total.
Pair of switches, DP3T on-on-on, one for each ignition:
Down ("Off") - power off, plead grounded
Center ("On") - power on, plead grounded
Up ("Run") - power on, plead ungrounded
One switch, DP3T (on)-on-(on), serves as test for both ignitions. I mounted this switch rotated 90* so the actuation is left-right rather than up-down, and it's spring-loaded to the center position:
Center - Power passes to both ignitions
Left - breaks power line to left ignition
Right - breaks power line to right ignition
(placard: "Pmag Internal Power Test - Left - Right")
I believe this is in keeping with the methods outlined by Emagair, though I think that any of these switchologies will need a checklist/POH section to explain the method of testing the internal power. ("Switch off plead of opposite ignition, then actuate test switch")