This really intrigues me. Are you in a PC or Mac? I use a Macbook and not quite sure how to set up an extra camera (I have a couple I could use). Would you be able to show a picture of your setup by any chance?
I am on a PC, and have almost zero Mac knowledge, but the most basic way to use a second usb camera, is just to plug it in, and it will show up in skype tools>video options, and you can select between them there.
An alternative is to try out the free version of ManyCam (available for Mac) and as long as ManyCam is on top (it can be dragged almost to the bottom of the screen to hide most of it) cameras can be switched (at least on PC) using ctrl+1 ctrl+2 etc.
Personally, I prefer Xsplit broadcaster, but I think it is PC only, at present. It costs $8/month, but for me it is worth it, as it offers many more options, and hot-keys still work even when completely minimised, so no need to remember to bring it back 'on top' each time.
My camera for displaying music sheets, is a Microsoft Lifecam Studio, attached to a desk clamped microphone stand (similar to an anglepoise lamp) which was around US$10 from ebay.
A bonus, is that I can place a tablet beneath the camera to display videos etc. without hogging PC resources.
Audio from the tablet goes to a small mixer, which also has a condenser mic, electric guitar, and digital piano connected to it.
Mixer output goes to a Komplete Audio 6 usb audio interface and from there into the PC.
( will need to have a major tidy up, before taking pics, lol )
Hope it helps.