Is there a way to join two DVDs made by spliting one using DVDFab? I need to create a single movie on one DVD and not add a second title after the first one.

I really never made it, but since no one else is answering you on this, maybe you can try to extract the M2V stream from the movies, first (e.g. with VobEdit) and merge (File___MPEG Tools__Merge & Cut) them with TMPGenc? Then you should do the same with the sound streams, and finally authoring the 'merged' video with the 'merged' sound (I'm just trying to guess..)

If you mean, make two into one complete movie, then ReJig will demux the vobs to m2v and ac3.
Do one, rename it, then do the second.
Something like tmpgenc tools, (merge) or Womble mpeg-wizard will join them together.
Once joined, it will need to be reauthored.
You may need to join audio separately in something like Goldwave.

Another thought...Virtualdubmod will accept vobs. You could load the first, then append, then resave as an avi, or frameserve.

If most of this sound's way too complicated, I think that DVDShrink has a join option as well, but I've never used it.