I’ll pitch in my two cents. With the software I have, and the aim of putting two dvds on one DL disk, here’s how I would do it. First I would reduce each dvd down to single dvdr size. I personally would use DVDRebuilder, CCE Basic as the encoder, but you can use whatever software you are used to—CloneDVD2, DVDShrink, etc. Just make sure the two videos combined will be small enough to fit onto a DL disk. You might have to make the output slightly smaller than default.
Then, I would open DVDLab Pro. There are three options when importing dvd vob files, you can join them into one large video, join them and demux, or use them as is and hope they are already compliant. Since you are coming from dvds, you should be able to use them as is, and this will save a lot of time. Next step is to make a menu so you can chose either video on the disk. DVDLab Pro will show you the output size, so if it is small enough to fit a DL disk, hit compile. If DVDLab Pro has an option to output to an ISO, I don’t know where it is (any hint would be welcome), so it will come out as standard Video_TS and Audio_TS folders.
I don’t trust Nero to burn to DL disks, so I would use DVDShrink to make the video into an ISO for me (using no compression), then use ImgBurn to put it on the Verbatim DL disk. Voila 7hrs of work and you’ve got a DL disk, with menu and two seperate movies on it.
I do something similar to this with TV captures, using single layer disks. I record episodes, wait till I have a couple of them, then combine them on one disk. But I never do this process of putting two dvds onto a DL disk. Too expensive and more than a bit inconvenient.
The other downside of this process is that DVDLab Pro is relatively expensive at $199. You could probably do the same thing with TMPGenc DVD Author 1.6 and save a lot of money, but I don’t use that program, so I don’t know for certain that it has the same capabilities.