X windows on SSI cluster

Is there a way to unite all of the various graphics cards in the nodes of a
cluster into a single X windows system?

I am aware that it is possible to assemble a group of discarded PCs into a
Linux cluster, with each node having its own image of the operating system.
I am also aware that it is possible to configure a cluster of multiple
machines to have a single system image (SSI) - making the cluster appear as
if it were a single multiprocessor system. has there ever been any attempt
on an SSI system to unite all of the graphics cards in the nodes into a
single integrated X windows system, wherein each node/card combination can
act as an X server? Users could then plug monitors and keyboards directly
into the nodes and have active X servers.

Knowledge of any software or project that has attempted this would be
appreciated. If there is some fatal flaw in this concept, please le me know
of that instead!

Much thanks in advance!