Increasingly conference rooms are being built with two screens, one oriented toward the left and one toward the right. I'm not talking about large conference halls, I'm talking about classroom size rooms. In the new stem cell building at Stanford there were two high definition plasma screens on the front wall of a moderately sized conference room. Their picture was sharp, but too small for the audience tor had easily. Eventually we had to replace them with a more traditional projector illuminating one large central screen.
But the major reason to work with only one screen is that the pointer is a critical element of a presentation. It connects the presenter's words with the visual signal coming from the slide. You don't want the audience looking at a screen without the pointer. So tell everyone to sit on one side of the room and only use one screen.
One nice feature for large conference halls is technology that allows a digital pointer arrow, operated from a laptop, to simultaneously illuminate all the screens. For room designers, this is a critical technology if you want to have more than one screen.