[CentOS] Help with multi-monitor Xorg stuff

I have an unusual use case.  I wonder if anyone can help.  We use a PC
with 8 HDMI outputs for powering a video wall in an operations centre.
We use two Matrox video cards, each with 4 outputs. "lspci" reports
these cards as" [AMD/ATI] Cape Verde PRO [FirePro W600]".

On an older version of CentOS 7, I used the proprietary AMD/ATI
driver. This had a utility (I roorget the name) that generated a
working xorg.conf file. I could them throw up the
"matchbox-window-manager" on each monitor in turn using
"DISPLAY=:0.0", "DISPLAY=:0.1" etc and then throw up a full-screen
chrome web browser in kiosk mode on each monitor after that.  This all
worked great.

In a recent version of CentOS 7, this all broke.  The proprietary
driver no longer works.  The good news is that the open source driver
seems to work fine with multi-monitor in Gnome for example.

My only issue is that I don't want to use Gnome across multiple
monitors.  I want to use matchbox-window-manager or similar, and
specify individual X screen (":0.0", ":0.1" etc).  How do I generate
an xorg.conf file with the new open source drivers?  Am I doing this
all wrong?

Thanks!