If I have burned (and saved the files) a dvd using the split feature,
can I somehow recombine them, so that I can then burn the dvd
onto a dual layer dvd ? ? If so, how ? ? Do I need to merge the
files first or just tell dvdfab which files to use and set output to
DVD-9 and it will know what to do ? ? ? Never tried to do this.

You can use Merge mode to do this, but there will be no menus on the merged output. You can use HDD files, files on disc, or a combination of both sources.

You would think if DVDFAB was the program that split the original DVD
in the first place, that it should be able to reassemble it back into 1 DVD
if that was so desired, without losing any functionality. As it is now, using
the Merge function, it seems DVDFAB can not even recognize files it created
itself. The file types do not show on the file open menu of the folders where
dvdfab saved the files. If this feature is really not in the program, perhaps it
is something that can be added, say when V6 comes out. It would be useful.
Only other choice is for the user to save the original DVD or a full file copy of
the original before dvdfab does anything to it.

I was exactly, to put the question:

At the time, i did use this program to:

First DVD: Menu-Extras and movie

Second DVD: Only the contunuation of the movie, from DVD1..

No chance of join them again?

That`s too bad....since now DVD9 is more cheap.....

Why not just re-rip the original?

Because, in this case, I do not have it.

For now, give DvdReMake Pro a try. It can merge previously splitted discs: http://www.dimadsoft.com/dvdremakepro/ht_merge1.php

It is good to know about that program, though it may be
hard to learn. And until I buy a Dual Layer Burner it will not
be needed at this time.

Aren't you supposed to "back up" only movies that you own

It was an instructional dvd that a friend of mine had.
The copy was made for him.