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.