Shortly after Christmas, that computer suffered a catastrophic disk failure (and as an aside, I hate LVM when that happens...). So I reinstalled, this time using the Ubuntu rather Kubuntu variant. This should allow use of Userful Multiplier --- a commercial multiseat solution with free two-seat licenses. The base package even comes via the Ubuntu repos.
I still had a couple of minor issues. One was possibly related to the Radeon card (as in: don't drive one monitor in dvi mode and one in analog mode but rather use both in analog mode via a dvi/analog dongle) so the live cdrom offered by Userful just went into a perpetual 'reconfigure, reboot, reconfigure, reboot, ...' loop. Another hitch was that their license manager no longer wanted to use the license key I had requested in November when I tried in vain to use Userful with KDE. And of course I wouldn't a new key as the home ip address hadn't changed... Now, with a newly requested key from another IP address, things appear to work at last using the default Gnome setup --- and the kids are back in proper 'parallel' use of their workstation.
All in all, Userful Multiplier is a nice and useful product especially as long as stock XFree does them the favour of no longer competing in the basic two-seat case.