The last post on this Google post hints that both Mac IIci internal and Mac IIci NUBUS Card videos can be used at the some time.

https://groups.google.com/forum/#!msg/comp.unix.aux/khA8Zho_cW8/exUDhTLJG7wJ

Thing is, can the system you are using do the same? I would guess so in the Monitors control panel if it is a later System. But that's my guess. Now to get 2 monitors see it at the same time!

Also, I found this Apple link that explain why some monitors don't work with a Mac and how to get them to work... Its a matter of grounding some pins.

http://support.apple.com/kb/TA40897?viewlocale=en_US#