somehow, it is now working perfectly. this was accomplished through the time-tested linux method of "i dunno -- i tried some stuff and then it worked."
it involved using the monitor configurator in kcontrol, sudo dpkg-reconfigure xserver-xorg, and nvidia-settings run as root so as to be able to write an /etc/X11/xorg.conf -- previous iterations failed to fix the top screen weirdness, or killed one or the other of the screens entirely (which made logging in fun; i'm currently doing the login from terminal, then startx thing -- makes me feel young again, because that's how we did it with kde-1.0).
anyway, thanks, everybody. you got the juices flowing and sparked the zen that sometimes is linux and especially x.