As Eltz said, and the fact DVD9 discs are pressed to make the dual layer as far as I know. The machines that make DVD9 would be mega expensive and only affordable by big production houses.
You can use a few programs such as <A HREF="http://www.dvdxcopy.com/" target="_new">DVD-Xcopy</A>, <A HREF="http://www.dvd2one.com/" target="_new">DVD2ONE</A> and <A HREF="http://www.dvddecrypter.com/" target="_new">DVD decrypter</A> which are all excellent in making a DVD9 dual layer DVD into one DVD5 (4.7GB DVD disc)and are my personal favorites. I also use the latest Nero burning software, real easy.
You can find more info here at <A HREF="http://www.dvdrhelp.com/guides?tools=&madeby=&formatconversionselect=&osselect=&howtoselect=6;30&search=Search" target="_new"> DVDRhelp.com</A> and also <A HREF="http://www.dvdrhelp.com/forum/userguides/144206.php" target="_new">here</A>.
Once you have figured out the way you want to do it you can get most of the tools/programs <A HREF="http://www.dvdrhelp.com/tools" target="_new"> here.</A>
<A HREF="http://forums.btvillarin.com/index.php?act=ST&f=41&t=326&s=24cd3646ec584db36d180555f4d4e868" target="_new">My PC</A>