First, if you have the time and budget, just buy Final Cut or Premiere and save yourself some headache. If you don't have the luxury of time and money and have to use iMovie '09, here is the best way to proceed:
Original long vid clip of talk
Original long aud clip of the simul-interp you want to overdub
If the video is in VOB format, which happens often with media people, convert it to mp4 using HandBrake.
Import the full mp4 (m4v) into iMovie. Handbrake will put a m4v extension on the file, and iMovie won't import that, so just change the extension to mp4 before importing.
Duplicate the project.
In the duplicate, select the whole clip and extract audio.
Select the audio file and save as WAV.
Import the simul-interp audio file into Audacity.
Import the WAV file of the extracted audio from the original video file.
Synch the two, being careful to orient the start of the resulting audio at the beginning of the audio extracted from the video clip. By doing so, you can record the time of the beginning and ending of the video clips you decide later and then use these times to cut the audio.
Silence the original extracted audio track.
Save as AudAll in MP3.
Back to original project, duplicate again so that the original audio is still embedded in the video clip as it was before you extracted it.
Go through video and cut it into clips. Review several times and make sure that this is the way you want to cut the video.
Use Excel or Calc to make a list of the clips and there lengths, calculating where the next clip will begin. Be sure to include the offset at the beginning that you get when you add titles. That can be Vid00.
Rename this project ClipAll.
Duplicate VidAll and rename to Vid01.
Delete everything but the first clip.
In Audacity, import AudAll.mp3 and use the times you recorded to select, copy, make new file, and paste the audio clip for video clip 1.
Check it, comparing the timing and content to the video clip you made inside project VidAll.
Save the audio as Aud01.mp3.
Import Aud01.mp3 into Vid01.
Fine-tune the syncing and make sure the end is okay, that you have enough audio to cover the end of the video clip. It is okay to go over.
Export project Vid01 as mp4 and then use the browser to upload to Youtube while working on Vid02.