Here is one solution for a powered “on” system.
It uses one generic automotive key switch and two toggle switches.
System has the safety of being off with the key off or removed.
You can jumper the IGN and ACC terminals on the keyed switch to have two contacts feeding the ignition selectors. The ACC terminal could also have a separate feed from the battery bus to eliminate a single point failure.
The “Mag” switches (really ignition selector switches) are turned on for engine running. The selector switches receive power from the keyed ignition/starter switch. Individual ignition be selected off for checking operation of the other side during engine run-up.
John Pitkin
Greenville, TX