1. Load Clip A into VirtualDub. Check the length of the video. Save the audio as WAV.
2. Open the WAV in an audio editor (sound forge, cool edit, audacity, etc). Check length of audio. Add silence if necessary so the audio is exactly the same length as Clip A's video.
3. Repeat steps 1 and 2 with Clip B.
4. Open WAV A in audio editor and add WAV B; save out full length audio A+B.
5. Open Clip A in VDub, append Clip B. Change audio from source to WAV, and select the file saved in step 4.
6. Select direct stream copy for video and whatever compression options required for audio, and save out new AVI.