No xinerama though - I prefer running two instances of X. Still, it works like a charm, don't have any issues at all.
Two instances of X, or one with two discreet screens? I use the latter - the larger primary screen (DISPLAY=:0.0) is my desktop-proper, the smaller second screen (DISPLAY=:0.1) is almost always running a full-screen Windows VM. So while I can't move windows between them, that doesn't matter all that much - copy/paste works fine, and that's all I care about.
The reason I do this, is that having one spanning screen sucks for virtual desktops - if I have something on the second monitor, I don't want it to disappear when I switch desktops on the other.
I run E16 (sometimes play with E17) - that's rather well suited to dual monitors (or more I suppose). No xinerama though - I prefer running two instances of X. Still, it works like a charm, don't have any issues at all.
How does that work? Do you need two of each input device (mouse,keyboard)?
Right now I have endless problems with X11 and multimonitor. All 3 of my screens are the same size and resolution, both are the same family Fermi chip (470 + 465), and yet there are tons of display driver/X11 bugs.
Xorg spikes to 100% CPU usage after a while, and gradually desktop responsiveness suffers until it is unusable.
Some KDE4 applications will only spawn on one monitor at a time
Some KDE4 localization is only applied on one monitor
Xinerama and all related solutions are deprecated. Deprecated! There's no supported way to run a consistent desktop across GPUs- NONE!
Two monitors are on one card, the third is on another. They're all horizontal of eachother. Sometimes, when moving the mouse between one X Screen (:0.0 and :0.1), the mouse will get "stuck". It will rapidly warp between the two X screens, and you are unable to move it from that edge. The Xorg process spikes and all other drawing halts. The machine has to be rebooted to be usable.
The first three are KDE bugs. The last one is a known server bug that you're right, should have been fixed ages ago. As for the fourth bullet, Xinerama is not deprecated; who told you it was?