From what I gather you can buy two tickets for one person. There was a travel journalist who actually recommended this as a cheap alternative to purchasing a first class ticket. On a cross country flight, you could buy 2 $250 seats in coach instead of a $1200 seat in F and get a lot of the leg room (kind of) that F offers.

One issue was that when the person did not show up for the second seat, the GA would give away the seat to a stand-by. To get around this, the airlines (of which CO is included) issue a second "special" boarding pass. There is a term for this special BP, but I forget what it is. I also do not remember what the process was to get this second BP, but it could be done.

I am not recommending this process, but merely recalling what I heard. It was a couple of years ago so I am not sure this policy is still in effect, but I can't imagine CO cancelling a policy that would allow additional seats to be purchased.