Dual layer DVD+/-R disks are still only write once; i.e. you have to write the whole contents in one go. If you want to merge two DVD Video disks onto a single dual layer disk then you will need to create a new VIDEO_TS folder which includes both the films. Check out DVDRemake or if you don’t want to keep the menus then you can use DVD Shrink in reauthor mode.
The software you are using is outdated; I suggest DVD Fab HD Decrypter for ripping the DVDs to the hard drive; just use DVD Shrink for reauthoring/shrinking. ImgBurn is the (free) tool of choice for burning disks especially dual layer. Oh, and we generally recommend only Verbatim or Taiyo Yuden dual layer media.
P.S. I think the full version of DVD Fab may have a merge facility, but I only use the free bit so I’m not sure!