It's not really that hard if you have the apropriate software. As you may have known, a DVD's duration is limited by the video's bitrate. But if you want to have a good quality DVD, you should limit it to 60 minutes in duration. So 30 minutes are just fine.
I'll try to explain using the software that I use, which is Adobe Encore DVD.
First, you need to copy the contents of the DVDs into your harddisk in order to be combined later.
Second, start the Adobe Encore DVD software, and create a new project.
Third, import the VOB files of the copied DVDs into the project as assets.
Fourth, create timelines by right-clicking and select 'create timeline' on the VOB files each
Finally, link the 'next play' in each timeline to the next timeline. And to make the movie loop, link the last timeline into the first timeline.
And now you're done! Export your project as a new DVD
Hope this helps
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