Ok, these are the 2 threads I used as references to get mine working:

http://www.dual-boxing.com/forums2/viewtopic.php?t=417
http://www.dual-boxing.com/forums2/viewtopic.php?t=877

This post details the command line options thing mentioned above:

Quote Originally Posted by zanthor
Quote Originally Posted by Kriwi
@zanthor
do you changed something in the maximizer .ini ?
I also use horizontal span and want to play both wow clients maximized.
Nope, command line options .

"F:\Program Files\WoWFrame\Maximizer.exe" -D 2 -R 1280x1024 --margin 0,512,0,640
"F:\Program Files\WoWFrame\Maximizer.exe" -D 2 -R 1280x1024 --margin 512,0,0,640

Thats my TopLeft and BottomLeft .25 scale screens for the right hand monitor on my system. I am in Vista so horizontal span is not necessary, so you would want to adjust and you wouldn't need the -D 2 commands. For the resolution use the full resolution of your horizontal span and then adjust margins appropriately.

So something like:
"F:\Program Files\WoWFrame\Maximizer.exe" -R 2560x1024 --margin 0,512,1280,640
"F:\Program Files\WoWFrame\Maximizer.exe" -R 2560x1024 --margin 512,0,1280,640

That should place two 4:3 windows on the right monitor.

One catch, there is some form of relation to the available video modes of the video card and what wow will start in... Under Vista I haven't been able to make 1280x1024 show up as an option in horizontal span... of course with no performance benefit it's not necessary.
edit: to clarify "F:\Program Files\WoWFrame\Maximizer.exe" -D 2 -R 1280x1024 --margin 512,0,0,640"

It's editing the target of the Exe, which you can place as a shortcut on ur desktop as shown above

The "D 2" part is Display 2, "R 1280x1024" is the resolution of that screen, and then use margins, play around with this a bit and u should get the hang of it. If not post the resolution onf ur screens here, and the desired effect and I'll see if I can get something you can copy/paste together... Hope that helps