I have a friend who does this. He and his wife by the 3 seats for two people so that he is ensured an empty middle seat on CO. So far it has worked out great for him. I didn't ask if he get FF credit for both seats but he's comfortable and happy.

He also offers them a deal if the flight is overbooked. He will give them 3 coach for 2 first class seats. Sometimes that works out for him as well.