Tiago Vignatti has announced he has revived work on the VGA Arbiter for Linux and will be attempting to push this code upstream in the Linux kernel, just four years after this arbitration code was first hashed out. The VGA Arbiter seeks to address an old problem where having multiple graphics cards that use the the legacy VGA interface with multiple X Servers could cause havoc with the same command being sent to both graphics cards instead of just to the intended adapter. This though is not a problem when the driver and/or hardware disable the legacy interface. However, for those affected, the VGA Arbiter fixes this problem for those graphics cards using the legacy VGA interface by controlling which one is accessed.
The patches that provide the Linux VGA arbitration code have been sent to the X.Org development list
with an explanation of this latest work by Tiago. He shares that running two X Servers in parallel now work (in a multi-seat setup) and secondary graphics cards work. The VGA Arbiter requires changes to the Linux kernel, X Server, and libpciaccess.
More information on the Linux VGA Arbiter can be found on the X.Org Wiki
and is described in detail on this page