ConvertXtodvd joins them for you. Instead od putting both AVI's in their seperate tracks, just drag the first file in then click on it to hilight it, then click the second icon down on the righthand side, "merge more files into this titleset" in the box that pops up there will be an add file button at the end of the extisting AVI. This will open an add file dialog box where you can choose the second part. Both parts will then be merges and made into a DVD automatically.
When doing AVI files with subs it will automaticall sort out both sets of sub files.