As you note - you can't use
TwinView and get X-compositing on your screens. What you really want to do involves multiple
Device entries in
xorg.conf for the same device and/or
mosaic mode. I went through this just the other day after rearranging the living room to accomodate a projector and it is not intuitive. I wound up doing it bit by bit with several false starts, but for one video card and two displays my
xorg.conf came out like this.
The one screen used to define metamodes is a sort of indirection. The
nvidia driver and/or
RandR extension adds together the display area of one or more physical screens and addresses that display area as a single virtual screen. Here is a link to the relevant section of
nvidia's readme. And here's what mine actually look like:
As you can see the two screens are one - but the resolutions per constituent screen are not evenly matched - so while that white bar at the bottom left shows up in a screen shot, it's not on my display. My display fills both of my screens. And here's a closer look at the
If you look at my own xorg.txt file you'll likely notice it is auto-generated. I got it to work by first powering off all of the displays but one and then, at a regular console, I ran
sudo nvidia-xconfig for a baseline
xorg.conf and, after starting
X, I opened
nvidia-settings and powered on my screens. It was not a scientifically perfect process, admittedly, but the result is satisfactory.
Another problem I had that you may or may not have was the sound EDID extensions passed over the HDMI cables from my displays. I use the analog out from my computer and there was another very annnoying few hours spent trying to convince ALSA/Pulse to ignore those extra
intel-hda-audio devices. So I rewrote the EDIDs to appear as DVI outputs - essentially disabling the computer's detection of the displays' audio sinks. If you do run into similar issues, I humbly recommend this little gem to automatically do the same.
Last, the Arch Linux wiki Nvidia page was very useful for me - there are lots of examples and detailed explanations there.