Are you paying cash, or using points?
I've read that if one books a first or business class tickets there's a
savings on a ticket for a companion. I;m not sure how that works but
many comments on this site have suggested that the price isn't worth it.
It depends on your route, and the dates. I used this feature to Japan from the US, and got refundable business class tickets for ~$600 more than economy, and $1500 less than regular non-refundable business class tickets were going for. It was a great deal.
If you decide to go this route, you can call Amex, and ask about the international airline program. The agent will be able to price out various options for you, and can usually hold tickets if you need some time to decide.