Horay! The solution was indeed to use XRANDR. This is how I went about using XRANDR with the FGLRX drivers installed. As a complete side note, this solved my flickering conky as well.
to disable xinerama, and disable all but one monitor
2. open /etc/X11/xorg.conf, in the DISPLAY subsection of SCREEN add the heading Virtual; Set the resolution to be the total possible Width and Height of all monitors combined
(I am using 2 1920x1080 monitors)
ex: Virtual 3840 2160
3. restart X or restart
to list all available displays
xrandr --auto to activate second display
xrandr --output (DisplayName-seen-with xrandr -q) --left-of (Other-DisplayName-seen with xrandr -q)
Play with this until you know which monitor is on which side. Horay! It works for now - until you logout or restart.
Making the changes permanent:
1. open /etc/X11/xorg.conf
2. look for the monitor sections with the names detected by xrandr -q (I had 4 monitors listed - but fglrx only manages 2 of them. I think the ATI driver interacts with a virtual monitor which sends the changes to your actual monitor, but I could be wrong)
Under that heading you are going to add a modeline, and a preferred Mode for your resolution
3. generate a modeline for each "Monitor" section with
. this is the resolution you want per monitor, not combined. ex:
The second line returned is your modeline
Paste your modeline under each monitor with a name you recognize from xrandr -q
Under your modeline add:
Option "PreferredMode" and then the first section of your modeline that is in quotations
ex: Option "PreferredMode" "1920x1080_60.00"
4. Since this is a multi-display you need to make sure that the monitors are not disabled, and that their position is correct
Check to see if there is a Option "Disable" "True" listed, and if so change it to "false"
For the monitor that you want on the right hand side, set:
Option "Position" "width-of-first-display 0"
ex: Option "Position" "1920 0"
That's it! Restart X (or restart your machine) to see if it worked!