Doubt it'll work. You have to call to book the companion certificates and once you have one booking, the next agent will see you and your FF# already on the flight. Even if by some miracle you got it done, you risk having all tickets canceled when TSA gets the flight manifest for screening and sees the double booking. That's a security irregularity. And it's possible, because you were basically committing fraud, that the airline won't risk to return your certificates or refund your money. You also run the risk of your FF account being closed. I definitely wouldn't try it.