Comment by mrweasel

Maybe I misunderstood something at the time, but the mga X11 driver was weird. My understanding, at least at the time, is that it is/was possible to provide an X11 driver that would work regardless of the underlying operating system, providing features such as dual-head support.

This feels intuitively wrong to me now and I doubt that's how it worked. In any case I dutifully copies the Matrox provided X11 driver to OpenBSD and enjoyed multi-monitor support.