> I'm not sure anyone has gotten a dual video card
> system to work correctly yet.

Numerous working dual graphics card setups have been reported here on MorphZone over the years, at least with Pegasos. According to pega-1 it should work with PowerMac as well:

https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=7459&forum=11&start=11

It's just that a Radeon/Radeon combination must be avoided, as jacadcaps says.