* Use VirtualDub to retrieve the 2nd audio streams from the two files that have 2 streams.
* Join these 2 audio streams together with an audio program suitable to the audio type.
* Rename the 4 AVI files with consecutive numbering in their filename. This will allow you to join the 4 AVI's together with VirtualDub's "append segment" feature.
* (Assuming that works,) You can then open the new concatenated AVI file in VirtualDub and use the audio tools to mux in the now-joined 2nd audio stream.
I could be off on this. It depends on if the AVIs can be joined and what the case is with the audio filetypes and such.