Robert_Charlton - 8:28 am on Jun 2, 2012 (gmt 0)
I haven't looked into the Livestream site very deeply... but to answer one of your questions, the device that's needed to allow you to use multiple cameras, as well as graphic inputs, is called a "video switcher"... or video switcher and mixer. Livestream may have all this integrated into one package, along with a web interface, but I'm not seeing exactly where.
Video switchers used to take up a whole studio. Now they can be put into small portable devices. Can't just be software, though there are some ingenious software packages... but hardware interfaces of all sorts are necessary to connect the cameras and microphones and switchers you use.
One camera is many orders or magnitude simpler than multiple cameras, though necessarily also much more limited.
Let me say that there's both art and technology involved in coordinating a live multi-camera video broadcast, or a live video taping session. It's more than a one-person job, and involves properly matched cameras, live color correction capabilities, appropriate technical specs for all pieces to interconnect, audio mixers, and color-calibrated video monitors... along with people who know how to use the tools. Pre-production of graphics is of course necessary.
The live Twitter feed suggests either a separate graphics input (and thus video switching)... or you might try projecting the Twitter feed on a screen and having the speakers interact with it that way... with the camera panning off to the screen. That can be pretty awkward, though, depending on how well you set that up, balance the light levels, etc... and how good the camera is.
You also need to understand the dynamics of video presentation and editing enough to do it in real time. I wouldn't try this with anything important without a lot of practice and preparation. There is a reason people hire professionals, even for the most apparently simple jobs, on such a production. I've found that every shortcut you take in such situations, unless you really know what you're doing (and are lucky), can ultimately be very costly.
Good audio, btw, is critical, and is the common failing of much of the web video I've seen.