I use two separate X screens and Xvideo is vsynced only to the primary display.
The DRM however supports syncing to secondary display as well (using
_DRM_VBLANK_SECONDARY).
I looked at the xf86-video-nouveau source, and it looks like it tries to
determine on wich display the window is by using the window position (in the
nv_window_belongs_to_crtc function). I think this method only works when there
is one X screen assembled of two displays, but it does not work with two
separate X screens.

Linux 3.15.10, Xorg 1.16, xf86-video-nouveu 1.0.10
GPU GTX 560Ti