I've used Xinerama with a setup of dual single-headed-cards.  As I 
recall, Mplayer played beautifully across both screens.  I haven't had 
access to a dual-headed card to try though.

I've since turned Xinerama off, and just run two displays, because my 
two screens are sized differently which leaves a "dead" area that not 
all programs (or is it the window manager?) deal with well.  (A program 
might open a window in a non-existent area of the entire virtual screen. 
 With a symmetric setup that shouldn't be a problem though.)

If it would help, I could re-enable Xinerama and try some specific tests 
for you.  The specific setup would be:
        Gentoo Linux 2.4.19-gentoo-r10
        Athlon thunderbird 1.2 Ghz

        screen 0: 1600x1200 on Nvidia Geforce 2 MX-200 (AGP)
        screen 1: 1024x768 on Voodoo 3000 (PCI)

It would be fun to see a wall set up and running :)

Best regards,
Shawn