Is it possible to back up a single magneto ignition system using an electronic system on the same spark plugs rather than having to change to two plg heads?
For a homebuilt or ultralight, yes. There are high voltage diodes that take a high tension lead from 2 separate sources with a single output, made for racing, that allow you to run a single plug from either of two sources, whatever they are. They are popular on Corvair aircraft engine conversions. You can run a mag and battery system together, or two battery systems, or whatever, with a one-or-the-other ignition switch.