Have you tried linking the first card to your monitor and the second to a dummy monitor with a single xorg.conf file? Scroll up to find my example of pointing my cards to a dummy monitor and compare the file with the one generated by
Tried. The cursor can still slide to the other screen/monitor, but the transition from one screen to the other seems to be without lag now.
Doesn't solve the problem, but at least helps out a bit.
Thanks.