While AT&T is currently advertising a buy one, get one free deal for Samsung's Galaxy S6, they are also doing so for the iPhone 6s, though it's not being advertised. For a limited time, when you purchase a new iPhone 6s, you can get another one on a second line for free. Of course, there are a few cavets.
The phones must be purchased on an AT&T Next installment plan, with one of them requiring a Next 24 plan. And you don't actually get anything for free, at least, not up front. Instead, you'll get up to $650 in bill credits for 30 months, enough to pay for one 16GB iPhone 6s on AT&T's Next 24 plan. So the charge will still show up on your bill, you'll just have an iPhone's worth of bill credits to pay for it. And if you want a phone with larger capacity, you'll need to pay up.
Additionally, it doesn't appear as though this deal is available online for iPhones. However, you can purchase your devices over the phone or at an AT&T store. The deal is available until December 31.