In March of 2015 @aplattner mentioned in this thread that support for G-Sync on mixed monitors would be available with 349.XX drivers, but in a subsequent one he mentions that there is severe limitations:
This essentially makes the feature unusable / broken. This forces users to either setup a secondary XDisplay and toggle between them when gaming or manually disable the other monitors. In both cases, you lose the benefits of a multi-monitor setup. Is there any plans to support this and bring G-Sync on Linux in functional parity with Windows (Supports both Fullscreen and Windowed G-Sync)?
I have also written up a Feature Request on the FreeDesktop bugzilla, but nothing has come of it.