
While it covers several handsets from a variety of manufacturers, the terms and conditions mention that both phones have to be from the same manufacturer. The value of the second phone will be credited to your account (up to $695) over a period of 30 months, apparently. So you're not getting the full amount right away. The credits will start reflecting on your monthly bill after three billing cycles.
It is important to note that AT&T offered the same promo on the Galaxy S7 and the S7 edge a couple of months ago, so this is a comeback of sorts. The offer is only valid until the 30th of June according to the carrier.