According to a recent mailing list sent out by Thomas, work is currently in progress to enable Ubuntu with Mir Desktop to offer Multi-Monitor Display support. This is as a result of numerous requests by Ubuntu users to support a multi-monitor screen. In the context of convergence, the Unity8 desktop environment will be powered by Mir. With that, the approach presented in is not going to work as X is not going to be around as the primary display server anymore Mir does not expose information about the global coordinate system (required to get window placement right) However, as the consistent placement of windows (in multi-monitor scenarios) has been requested numerous times for the current version of Unity, we decided that Mir should offer support for it out of the box.
Mir 0.14 recently released, is the next generation display server targeted as a replacement for the X window server system to unlock next-generation user experiences for devices ranging from Linux desktop to mobile devices powered by Ubuntu. The primary purpose of Mir is to enable the development of the next generation Unity Mir 0.14 highlights: ABI summary: All clients and all servers need building; Mirclient ABI bumped to 9 Mirserver ABI bumped to 32 Mircommon ABI bumped to 5 Mirplatform ABI bumped to 8