VirtualDubMod is outdated and you should use plain VirtualDub: http://virtualdub.sourceforge.net/
That said, if you want 2 tracks to play together, you'll have to mix them, with your NLE perhaps (vegas/premiere/magix/whatever you're using... even wmm should allow you to do this with the audio).
You can mix them within avisynth too, if you wish: http://avisynth.org/mediawiki/MixAudio
One tip: keep in mind the levels when mixing 2 different audio tracks together, even if one is just voice, if volume is too high on both, it'll be way too much to take and will kill the ears. >_<
If you use the sample avisynth command in there, then the loudness of the audio when mixing is kept in mind.