The multi (multi == 2) seat setup as described earlier has been working very well for the last year. However, I refrained from upgrading. Just in case.
One more thing I forgot to mention - in my setup the kernel mode setting is OFF, using "nomodeset" kernel command line parameter.
I am going to set up a test system on a separate hard disk and each time I want to upgrade, I will upgrade the test system first. If it works, the system proper should work, too.