DVDFab produces DVD-Video compliant output and the developers have been reluctant to allow user control that would produce output that you couldn't burn to a blank disc due to size (support issues I think). I would suggest that you decrypt and write the original discs to your HDD using DVDFab, then use DVD Shrink to merge the titles that you want, using the DVD browser tab in re-author mode. You will need to set the preferences in Shrink (Edit->Preferences) to a custom output size before starting the merging. Caveats: there will be no menus and the non-standard folders may not play on some players and devices that strictly construe DVD requirements.
