The short way is to use DVDShrink in "Reauthor" mode.
Drawback :
- quality of the image of course but as you don't want to reencode, you probably already supposed that.
- you loose the menu (no solution to keep the menu exists).
- you have to use the "title" select mode of you SAP to go directly to the next DVD. With some players, this is not convenient.
http://www.dvd-guides.com/content/view/46/59/
(other guides using other tools for the same thing :
http://www.dvd-guides.com/content/category/3/78/59/)