I swear by
dvd remake pro. I use it all of the time for a number of operations.
Combing dvds in it is very easy as it also preserves the full menu structure of the dvds you are working with.
it goes like this
1. input first dvd (browse to the video_ts.ifo file)
2. input second dvd (same as above)
3. go to the tools tab on top and click it, then click "Merge" and you will see both dvds listed. Hit ok and you are done
export your dvd and burn (full disc, NOT modified files).
Remake pro creates a new title menu that lets you select disc 1 or 2. Using the top menu button on your remote brings you to this base menu
if you are more experienced you can over ride the menu and truly link both dvds as one, etc