UA and AS used to have these companion fares for travel on the west coast. They often had restrictions, like a Saturday night stay or the companion might not receive FF miles. (AS was more likely to give FF miles than UA to the companion. Guess which one I booked? ) These were available to the general public and the most important criterion was that the 2 people must travel together on the same flights.

I haven't seen these in a while, but did read in a travel newsletter that they seem to be coming back. Sorry I don't have more details for you.