Do you shoot videos with a camera or camcorder?
"Is there a way to add two audio clips to a Windows Movie Maker video which overlap each other?"
Windows Movie Maker only supports two audio tracks: the audio track recorded with the video is one of the tracks; the other movie maker audio track is the Audio/Music track. In your case you need an additional audio track, so that you can layer your two audio files on top of each other. If you plan to do this regularly , the easiest thing to do is graduate to a more complete video editing suite, like Pinnacle Studio, Roxio Creator, or Sony Vegas. If you're on a tight budget or you'll only do this one time, you can combine the two tracks in another application, save out a single audio file, and import the combined audio file into Windows Movie Maker.
One easy to use solution to combine two audio files is Audacity, which is a free mutli-track audio editing tool. After installing Audacity, launch the application, then go to File > Import > Audio and import your first audio file. Repeat this step again to import the second audio file.
Once you have the two tracks imported into Audacity, you can edit them to adjust volume, trim unneeded portions of the audio and lining up where you want the overlap to occur. When you're finished with the editing, choose File > Export, choose a file name and save your combined audio track. Drag and drop the file into Windows Movie Maker and add it to the Audio/Music section of the timeline.
