I believe I just responded to this issue in anothet thread. Below is a copy of that response.
First, the 'DVD Copy' option will never allow you to create a 'Multi-Session' disc, no matter who's software you use. Any copy process will automatically close the disc upon completion of the write.
It sounds like what you want to do is take say 5 DVD-Videos disc, and combine them into one DVD-Video disc, is this correct?
If it is, then you can do this with the Nero Recode software. In the Nero Recode software that comes with Nero 6.6 and Nero 7, there is an option called "Compile a DVD". And with this feature, you can select various VOB source files and burn them to one DVD.
However, there is one bug that we are still working to resolve. And that is this feature will only accept one DVD-ROM drive as a source for VOB files. All other VOB files will have to be stored on a hard drive or Network drive.
We hope to have this resolved in the future so that you can select multiple DVD-ROM drives as the source for 'Compiling a DVD'.