Wade wrote:

I've been taught this shtudown procedure, too.
But, what is the magic about turning them BOTH off, simultaneously?

If, at idle, you turned off the left mag, and noted an rpm drop,
then turned off the right one, and also noted an rpm drop,
then doesn't that confirm the same thing as turning
both of and feeling for the engine to start to shut-down?

The procedure you suggest does not test the OFF position of the switch itself.
These cheesy switches have been known to fail that way.