Their webpage suggests they can handle multiple devices connected to the one PC, so I was wondering whether they had another sensor out of shot above the scene pointing downwards.

Actually - scrap that. A better hunch is that it's not structured light at all, but actually an electric field sensor. See this Quora answer (disclaimer, by someone who "knows shit all about this"): http://www.quora.com/Leap-Motion/What-is-the-technology-behi...