what you are talking about is a "two sided"dvd. a dual layer dvd is two layers on one side. there are also dual layered two sided dvd's, Terminator 2 is one of these.
the information you are looking for is easily found here in this forum.
try this http://ifoedit.wh.fr0zen.com/joinclip.html#joindbl
it worked for me for Goodfellas,a two sided dvd,and Fellowship of the ring extended edition,a two dvd set, i joined the two dvds to one dvdr.
remember,all dvds are different,each one you do may need to be tweaked in a different way with ifoedit.