Not real sure I understand your question but I’ll take a stab.
when you put in a blank D/L media to burn to, DVDFab, only see’s it as a single,
if this correct, then go into common setting ( the green button top bar with a check )
go to left column- DVD to DVD …> default output size, and change to DVD9
then on main page bottom left, Quality drop down, again change to DVD9
hope this helped if not let me know.