The two displays are entirely separate subsystems, each capable of only 16bit Color. There is no 24bit Card available for any of the PBX bridged, NuBus Architecture PPC-PBs. The addressing of the I/O bus is split, 16bits to the PCMCIA Card Cage and 16bits to the Video Card Connector.

I'm going from memory here, check the DevNotes, YMMV.