There are free ripping tools to get the movie onto your hard drive; I haven't tried this tool but it sounds like it would work:
http://www.winxdvd.com/dvd-ripper/
There's also plenty others out there:
http://lifehacker.com/380702/five-best-dvd-ripping-tools
Once you have the two movie files (avi, wmv, etc), if you have windows vista or 7 download windows live movie maker (I think XP had a different movie maker), add both the movie files, and let it spit out a combined movie as a file or a DVD.