paolo@spring wrote: How can I use a second external webcam with my MacBook Air 11" 2011?...
Plug it in and launch the app you will use to run it.
paolo@spring wrote: ...Are there specific compatible webcams?...
Yes. In general, any current USB model (will meet the "UVC" standard) can work on any Mac running Mac OS X 10.4.8 or newer.
More specifically:
(1) You have a choice of many Mac-compatible USB cam. Some examples of current compatible cams are described at http://www.mac-compatible-web-cam.com/.
(2) If you want a wireless cam, consider http://ecamm.com/mac/webcam/bt1/
(3) OS X already has everything it needs for Mac-compatible cams. I recommend that you not add any OS X software that comes with your chosen camera to your OS X Mac, at least until you have tested your new cam without adding anything.
paolo@spring wrote:... Can both cams be use at same time?...
Multiple cameras can be connected at one time, but asking your Mac to actually use (capture and process) more than one video stream at a time might cause processor and data bus capacity problems. The normal method is to connect your cameras and switch between them as needed, actually using only one at a time.
paolo@spring wrote:... How would I choose which cam would be the active one, for example with FaceTime or Skype?...
The software you use to operate the cameras controls that. For more info, start with the answer in this topic link.