If the displays are attached to different GPUs, the only way to synchronize stereo across the displays is with a G-Sync device, which is only supported by certain Quadro cards. See Chapter 30 for details.
FRAME LOCK: Frame Lock involves the use of hardware to synchronize the frames on each display in a connected system. When graphics and video are displayed across multiple monitors, frame locked systems help maintain image continuity
to create a virtual canvas. Frame lock is especially critical for stereo viewing, where the left and right fields must be in sync across all displays.
In short, to enable genlock means to sync to an external signal. To enable frame lock means to sync 2 or more display devices to a signal generated internally by the hardware, and to use both means to sync 2 or more display devices to an external signal.