You don't need to join them, the tutorial I assume uses DVD2SVCD which in turn uses AVISynth so all you need to do is
Part1 = AVISource("part1.avi")
Part2 = AVISource("part2.avi")
The above will also ensure a synced result even if the audio and video are different lengths, unlike joining with VirtualDub.
For instance the first file could be:
and the second
Now both would play perfectly on their own, however if you join them with VDub you would get
which is obviously not in sync. Using ++ as upposed to just + with AVISynth will make sure that the results are synced by adding/removing audio to match with the video. Plus you don't have to create a new unecissary AVI.. you can just encode straight from the original parts.