I’ve been using two monitors with two graphics cards on my computer for long time. I’ve got another spare old CRT monitor so why not to utilize it? I connected it to a free VGA port on one of the cards and started experiments.
Compared to the two independent graphics cards setup, using two monitors connected to a single card handling a single desktop was relatively easy. By default the monitors show the same screen. I tried to setup Xinerama but these attempts have always finished with X server segmentation faults. After some more searching I’ve found that the proper tool to use, other than Xinerama, is XRandR. I just had to upgrade the xrandr utility to a newer version. Then it was easy, using xrandr one can play with dual monitor setups on the fly, without needing to restart the X server (which is equal to reboot on my dual card machine). And StumpWM works great with XRandR, I had just to figure out that the second monitor screen is nothing else than another frame.
The only problem is that the whole XRandR setup is limited to the total possible graphics card resolution, 1920×1200 in my case. So the second monitor is limited to 640×480 resolution in my setup. But it’s still well usable for tasks like watching TV while doing something else.