Welcome to the forums carldouglasx.
There are several different ways of doing this, depending on what format you want the movies in. If you want dvd-video, that will play on all dvd players, the quality of the video may suffer to some degree...hard to predict depending on the length of the movies and the amount of action within them.
If you are using commercially made dvds, you'll need to decrypt them and rip each one to the hard drive. I suggest just using the main movie, and pare down any extra audio and subtitle tracks. DVDFab and AnyDVD are the two main decryption/ripping programs.
Once on the hard drive, you can import both movies into AVStoDVD or DVDFlick and create a new dvd with a menu to select whichever one you wish to watch. I strongly suggest making this a DVD9, which means you'll need a double layer dvd on which to burn it. Only use Verbatim brand double layer dvds, and I recommend using ImgBurn to do the actual burn to a disk.
If you don't mind converting the dvd to another format, I'd suggest using MakeMKV. It can decrypt a dvd movie and convert it to H264 video in a mkv container file. An entire dvd movie that takes up 5-6gb in mpeg2 (the codec used in dvd-video) can be reduced to 700mb and still have excellent quality. You could fit 6 of these movies onto a regular single layer dvd. The problem with mkv files is that very few players support them, though they are easy to play on a computer with VLC or Media Player Classic HomeCinema.