Hey folks, just to let you know, both myself and Joseph Kain have dual voodoo3's working. One card is AGP and the other is PCI.
The somewhat hacky way I got it working was to install the XFree86 4.0 binaries from XFree86.org, and then put a new tdfx_driver.o in the appropriate place. Xinerama works fine, though it's picky about which monitor is on the left. My experience is having the primary monitor on the left works best.