I got fed up with Ubuntu's crappy UI, so I went back to basics, but the
default new window manager stuff gave me a small, mirrored desktop across
both my monitors.

To save someone else the grief, here's a quick how-to:

1) Add a new window manager for rio.  Your paths will vary.
~ $ cat /usr/share/xsessions/rio.desktop
[Desktop Entry]
Encoding=UTF-8
Name=rio
Comment=
Exec=/home/plalonde/plan9port/bin/rio.sh
Icon=
Type=Application
~ $

2) Fill in rio.sh to use xrandr to set up your display, and run whatever
init you prefer:
~ $ cat plan9port/bin/rio.sh
#!/bin/sh
xrandr --output LVDS1 --mode 1600x900
xrandr --auto --output HDMI1 --mode 1920x1200 --right-of LVDS1
/home/plalonde/plan9port/bin/plumber &
/home/plalonde/plan9port/bin/rio
~ $

Replace LVDS1 and HDMI1 with the names of your video outputs, as reported
by "xrandr -q"

That should be enough for demuddling the desperate and sufficient for the
method to be indexed for posterity.

Paul