I should be wearing a helmet!
Wow, thanks for the great reference, Deano!
The easiest solution I found was to install 13.1x64 with just one (with my old, known-supported GTX 650) card in the lowest numbered PCIe-16 slot. Then I installed the NVIDIA driver from a console login. (This needed a couplel of reboots and a reinstall of the driver to get fully installed - apparently it has scripts to disable nouveau, but reboot needed…) Luckily, my GTX 750Ti uses the exact same NVIDIA “.run” driver installer. After all reboots, with the GTX 650 was running fine, then another power cycle let me drop the GTX 750Ti into the second PCIe-16 slot and when I restarted everything was running fine.
Come to think of it, I was using a later-numbered “.run” installer previously - I guess I just assumed that the newer numbered driver files were backward-compatible, but maybe not - maybe that was part of my problem all along! But there’s still the nagging issue that the 13.2 installer threw weird graphics on my GTX 650 card, when I tried the above prodecure, and the GTX 750Ti is definitely unsupported!
Now that I’m stably up and running for 24 hours (fingers crossed - both GPUs happily processing and fast system otherwise), I’m thinking about an online “dup” to 13.2, but the new kernel would require a NVIDIA driver reinstall, and the automated NVIDIA installer scripts may not work with the 13.2 nouveau install. So, for now at least, I’m OK since 13.1 is going “Evergreen.”
Thank You for all the information, and HAPPY NEW YEAR!!! Espresso morning, here 