As I understand the following airlines operate the companion ticket scheme from the UK. They are;

AC
CP
AA
CO
UA
LF
EK
SA
AZ
NH

I must admit that having looked in to this offer it seems to be almost worthless as the price of a full fare ticket is so high. The only possible use I can see is for business people wanting a flexible ticket that they can bring their OH on. Can anyone contradict me on this? Have they made use of this scheme?