Multiple recordings into one

The short answer is you don't.

The long answer would be that you make an off-line version (FLV or, if you have paid for the ability MP4) and then combine them in a video editing application. Alternatively, you can pull the audio from the source files of the recording (add /output/yournamehere.zip?download=zip to the recording URL) and then use Presenter or Captivate or whatever the appropriate tool is to add the audio back into the presentation(s) and edit them apart/together as you see fit.