This is a very difficult topic. The major reason is the H/W. When we tested the multi-display mode we identified that 
two Radeon cards would not usually work together. The symptoms are just like you described -- driver's crashing, system 
lock-ups. But we came across some Radeon configurations that would work. I do not have the model numbers, but I think 
those were an AGP and a PCI radeon.

In general it is very difficult to find a working combination. We have came across of a few. For example a Radeon and a 
Carmine (which doesn't have a VGA BIOS) will work together pretty well -- but again, not always. For example some x86 
systems would not even boot when the cards are plugged-in.

You may have a better luck if you get a specialized graphics adapters (of the same kind I assume) that can work 
alongside in one system. Most likely you will need a custom driver too.