so that if for some reason both become free, I can be sure that I put the primary back in my mouth.
Seems to me that you're looking for a solution to a problem that doesn't exist. If your reg comes out of your mouth, and for some reason there are two regs available because "they both came free" simply put either one in your mouth first, then figure out which one it is. At that point you can swap regs if necessary and re-stow the other one.
That said, a bolt snap on your primary - the main purpose of which is to clip it off when not in use - will have the added benefit of allowing you to differentiate your primary reg from your pony reg. (Providing far more utility than a wiffle golf ball, too.)