Hmm.. I thought it was more like OpenFirmware doesn't initialize several Radeons, only one, and that's why you'd need different cards, but I guess Geit knows better.
But anyway, basically every Amiga system supports several gfx cards (CGX and P96 on both classic and next gen). You just get screenmodes for each gfx card and you can select programs' prefs which mode (==card) they use. For example all MUI programs can freely be configured to use separate screens and almost all classic programs have screenmode settings. We've seen demonstrations of three PIV cards on three monitors on a single A4000 for example.
