Open the 1st AVCI with VirtualDubMod. Do File___Append segment and load the 2nd. Be sure that Video__Direct Stream Copy is selected. Then do F7.
Load the saved 'merged' AVI. Do Stream___Stream list--->Add and add the AC3 stream. Again, Video__Direct Stream Copy selected, do F7.
The final movie will be an AVI with that file as sound (how did you get 2 AVIs and a joined AC3?).
If the sound goes aout - of sync, play with BeSweet adding some ms of silence. If you need to do more, you need to save the WAV and edit it. Then, compress it to MP2 and again .... F7.