xinerama and AT3D

Has anyone experience on how to make PCI cards based on Alliance AT3D
work with Xinerama?
My system has an AGP RIVA 128ZX which has been functioning properly so
far as the unique video card.  I tried to add two different AT3D-based 
cards, a Colormax and a ViewTop, but to no avail.  Starting X makes the 
AT3D card's BIOS show its banner on the second monitor, and then 
completely locks the entire system.  Even ssh connections cease to be 
available.  However, I was able to set up Xinerama successfully using a 
S3 VirgeDX in the same PCI slot 0:12:0, I had sed for the AT3D cards.
I searched google for reported problems with AT3D / Xinerama but I 
couldn't find anything relevant.  Actually, Google returns (among other 
matches) dangling links to the Xpert pages at

http://www.xfree86.org/pipermail/xpert/...


I'm using Mandrake 8.2 with XFree86 4.2.0, and of course the "apm" 
driver for the AT3D.

Thanks

       Stefano Lodi