If we add the recommended dual video card, does that override the internal video card? In reading the foldback information, seems like I'll be using one out for the front projector and one out for the rear projector. So how do I get the computer monitor to work?

It depends on the specific computer. Many will automatically disable the onboard graphics when a discrete graphics card in installed. But that shouldn't matter as any graphics card that meets the System Requirements for EW will have at least 3 outputs, with many having 4 or more. You just might need to use some sort of ACTIVE adapter to use all of them. And yes, you need an ACTIVE adapter, not a passive one. The description of the adapter will identify it as ACTIVE or passive. If it doesn't specify one or the other it is a passive one.