It's worth noting that this deal is valid for only the downloaded version; the retail version does not have the same price discount or special offer. The deal doesn't technically reward you with an immediate copy, instead providing a $9.99 promotional credit that can be redeemed only on a second copy of TERA. This credit is valid until October 15th, but the promotion ends on September 22nd.
If you and a friend have been tempted to give TERA a shot, now is obviously the time to jump in, although you may want to wait for a day as the game is merging down to three servers tomorrow. In fact, En Masse has just officially announced the names of the new servers and revealed additional details about the merges on the TERA website.
[Thanks to potatochobit for the tip!]