There really is no practical way to do this though.

Option 1: Make sure the CP holder's boarding pass is scanned first. But what if the CP holder is running late, or the companion has a higher boarding position? Now the companion has to wait to board, and loses out on their position.

Option 2: Make both board together (same issues as #1 )

There is an obvious workaround for this that I won't share here, but needless to say it would be easy for someone to circumvent any "simple programming" designed to catch CP cheaters. The easiest way to police this is what southwest is doing right now - physically verifying both people are on the plane in the event just the companion's boarding pass was scanned.