The AMEX Platinum International Airline Program is the feature you've heard about.
It used to offer a companion ticket (free, except for taxes, fees, etc) when you purchased a Business Class or First Class ticket through their Travel Agency. The paid ticket was (slightly discounted) full fare. Full fare benefits, including no penalty cancellation, accrued to both tickets.
Today, it offers discounted tickets in PE, Business, or First. Each ticket is a paid fare.These are not full fare tickets and accrue the benefits of their fare class.
I've used this program in both it's iterations and have been well pleased. Even under the old program, I saved a few hundred dollars in comparison to the cost of two (for example) Z fares. With the new program, my savings on two fares to Europe, compared to those I've found independently, have consistently surpassed the cost of the cards. I find it most efficient to find my specific itinerary and price it before calling (you must call to purchase through the IAP). There are also significant Membership Awards - 5 points per dollar - for booking with AMEX Travel, whether using the IAP or not.