apart from v.dub whats the best tool to use when mergeing two seperate files together wether it be mpeg,avi.......etc?

i want to know coz im encodeing two avis into dvd useing tmpg then i need to merge them and place in dvd author.
am i doing this correctly?
ive also noticed while tmpg is encodeing the avi file its also encodeing a deperate wav file?
it only seems to do this when im encodeing in dvd mode.