If both your movies are already in dvd form, or they are mpeg2 in dvd compliant configuration, then what I would do would be to author a dvd with a menu that you could use to select either movie. For that you need an authoring program, like TMPGenc Author 1.6, DVDLab Pro, Nero Vision, the Ulead programs, or something similar. These commercial programs usually have a free demo that you can try out, and I really recommend using one of them, as they are easier to use and more polished.
There are some free authoring programs, like DVDAuthorGui. Check out the program here and read the guide for using it. There is also GUIforDVDAuthor, here
You don't seem confidant that your movies will fit onto one dvd. If that is the case after you make this new dvd with the new menu, you can always run it through DVDShrink to make it fit onto one disk. It will transcode and be reduced in size like any other dvd that you have used in Shrink.