But two cards are not needed. As the machine is running a standard Kubuntu setup, I just followed this excellent three-part tutorial for Ubuntu multi-seat setup which describes the process using nothing but standard Ubuntu software. From setting up a 'big desktop' spanning two screens (which is easy enough using one card via the vga and dvi outputs), it is fairly straightforward to modify the gdm.conf setup to spawn two gdm greeter instances using the Xephyr nesting xserver.
So far, all is well. We'll see what possible shortcomings we will find. The GL extensions are not supported, so some eye-candy will be unavailable.