I have had the same problem
The solution is quite simple
When you create a DVD file with WinAVI, what the software does is create the entire DVD structure consisting of 2 folders:
AUDIO_TS
VIDEO_TS
The AUDIO_TS folder is empty
The VIDEO_TS folder has 5 files:
VIDEO_TS.IFO
VIDEO_TS.BUP
VTS_01_0.IFO
VTS_01_0.BUP
VTS_01_0.VOB
When you create a second DVD file with WinAVI, it creates another set of similar files.
If you try to import both DVD files into Nero, it will result in an error because it does not know which set of files to take as each one is an entire DVD disk structure in itself.
To author a DVD with several files you can get DVD Shrink 3.2.
Get it here *www.dvdshrink.org/
Start DVD Shrink
Choose Re-author
Now navigate to the folder where your DVD file was stored.
Instead of seeing VOB, IFO and BUP files, there will only be a single video file named Title 1
Add it to your compilation
Repeat the steps to add as many files as you want
When you are finished, click on Backup
Choose your DVD Write and start the burn process