There are two functions that seem to be about the same, but are not.
Rendering - this is the process of creating an AVI file for playback. These will be in the Media Cache. They are not used for the Transcode (next) process by PrE, and are only for playback.
Transcoding - this is the process of encoding the Timeline to MPEG-2 for use in the resultant DVD-Video.
In your case, probably the easiest way to get an additional duplicate of your DVD-Video would be to just use the freeware utility, ImgBurn, to duplicate a finished disc. If you have two drives/burners on your system, this is just a simple write and takes little time. If you have one burner, then the program will need to copy the material from the source DVD-Video to the HDD, and then write from that to a blank disc in the same burner.
What you describe with the need to re-Transcode the Timeline, is common. If I Burn to Disc from an Encore Project (my authoring program), and then want to Burn another later on, the Transcoding will need to be redone. When I have multiple discs to Burn from a Project, and suspect that I might need to increase the number of discs, I first Burn to Folder, and then just use ImgBurn to create the physical discs from that Folder (VIDEO_TS). If I get a call for 3 more discs, a week later, I just use that folder.
I'm most grateful for that. It's a pity that the humorously called Help
files don't mention that difference. I've certainly learnt to burn
several at one go under "Share" but sometimes I get caught out. I
normally use Nero for disc burning/copying.
You are most welcome. The Nero utilities should do the same job that ImgBurn will, depending on the Nero "suite" that you have. I've used the disc duplication utility in Nero in the past, and it worked well.