Ok, I'm almost there. I found a typo in my xorg.conf - actually, I had
copies one from someone out there that said it worked. Fixing that, in the
ServerLayout section, and I've now got the two screens, side by side, and
reachable. My two remaining problems: first, if I scroll around, it
appears as though X thinks that the screen is bigger than the monitor, and
it moves visible off the top of the monitor, leaving an L of black. The
other is, if I log in as root, and try to use System->Display, it tells me
RandR is disabled; meanwhile, in /var/log/Xorg.0.log, there are the two
entries, one for each displayport, that say "ignore the following randr
disabled message....
mark