Sounds like a double sided DVD5 then.
Yep you can do it, but I still reckon you wont be happy with the quality, even at 50%, but try it and view it in Power DVD, before spending too much time on it.
If you ARE happy with the quality, you will need to rip/compress each side to say 2100mb (DVD Decrypter & Dvd2one), then demux each side (VOBEdit), to get an m2v and ac3 file (movie & audo). Next you will need an authoring program like SpruceUp to create your menu and produce DVD compliant files ready for burning.
It's a lot of learning and work and could take you many hours first time round. IMO it would be much easier to just buy a double sided Ritek or use 2 disks, that way you won't degrade the quality.
There are a number of guides around, try DVDRHelps website.