AFAIK, most DVDs are encrypted to prevent disk->disk copying. However, there are applications that break this encryption and compress dual layer movies to a size compatible with single layer DVD disks.
Now, these acts may or may not break laws in your country of residence, so a more detailed discussion is likely to risk a red flag.
However, were you to go to a site like versiontracker.com and enter words describing what you want to do, there are a number of open source programs which should be listed.
soi la, soi carré