There are a number of applications that will let you rip the audio from a DVD. Here are a few I found using Google:http://www.imtoo.com/dvd-audio-ripper.htmlhttp://www.dvdtox.com/dvdaudioripper.htm
Otherwise use DVD Decrypter (Google for download link.. its free) and enable stream processing. From there, you just select the audio stream that you want to extract. You can do the same thing to rip the video from the other DVD.
To put it together, you'll need a video editing application. Just add the video and then the audio track. The hard part is going to be getting the audio and video to match up (talking, etc).