If the dvd doesn’t have one of the more advanced protection schemes, you can use DVDDecrypter in ISO Read mode. You’ll get an ISO and a little mds file that will contain the layer break information. Load the mds file into ImgBurn and burn to your dual layer dvd. Lets hope you only bought Verbatim DL media made in Singapore.
If you run into a dvd that DVDDecrypter can’t handle, use DVDFab HD Decrypter to make the ISO. Load it into ImgBurn and let that program set the layer break.
Either method works. Saugmon’s link is for Imgburn. Look in the ImgBurn forum and you’ll find some guides on using the program.