First, use Alcool 12%: View-Options-Virtual Drive, set up two (or more) virtual optical drives, return to Alcool 12% main window, and add two (or more) image files (such as iso and mds) to the list. Then choose a virtual CD-ROM drive and add another image file in the same way.
Second, use dvdReMake pro(v3.14) and follow the instructions of the above website, "Import DVD-Merge DVDs-Export" to complete the regeneration of the newly merged DVD. (The regenerated video dvd file has the merged menu and the original menu perfectly)
Third, burn the regenerated file with nero. (by burning DVD video)