DVD Remake Pro (and awesome program I couldn't live without) is a good answer: http://www.dimadsoft.com/dvdremakepro/ht_merge_full.php

If you're looking to make it a single non-stop movie, you will need to rip the movie from both sides, merge them in your favorite video editing software (Womble makes several nice alternatives, including the $20 Mpeg2VCR which lets you do simple straight cuts and joins of mpeg video) and then reauthor it back to a DVD.

Lots of ways to do the second method from a simple rip, join, author, burn, to as complicated as rebuilding onto the original DVD.