Good thinking SupraSabre.This is the simplest and cheapest idea... most just file a small notch into one side of the key as a reminder of which side is which.
The ignition keys, at least in most cars, actually need both sides of the key, but this approach would likely work for gas/panniers/pocket locks. I'll cut a key on just one side for my own bike and see if its sufficient to work the ignition, and report back what I find.