As I've spent a good few hours researching solution for the problem, I just share the solution as it seems to be rather unobvious:
The Xinerama support for the stock driver is not working, when starting X it comes up with the error
>MGA(0): Unable to map BAR 0. Invalid argument (22)
Some great folks have solved the problem for Ubuntu here: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-mga/+bug/292214
Though the patch found there for some slight differences in the structure of the mga driver source from arch and from ubuntu won't work.
I've manully patched the driver (version 1.4.11), and it does work now.
You can download the patched source of 1.4.11 from here.
Also a working xorg.conf is found on the page.