I used to work for a distance learning program, that would connect through the internet to other DE sites. The setup we ran was two projectors in the front (one showing the presentation, one showing whatever DE site was currently active) and then two in the back (same thing). It did allow the speaker to both look at the audience while still looking at the slides and look at the camera while looking at the image from the other DE sites. Otherwise the presenters have a tendency to turn around and look at the de site on the front screen, which gave the camera at the site the back of their head.