firbec, take a look around http://www.videohelp.com for information on DVD-Video and tools that can be used to manage them. This section on the file structure of a DVD-Video disc tells you something about .ifo, .bup, and .vob files:
http://www.videohelp.com/dvd#struct

The thing is that each DVD-Video has it's own menus, chapters, and such so to combine two of them into one would require some very high end authoring software to create a new top menu that allows you to choose between the two disc's main menus.

If you are fine with just having the videos on one disc, without the menus, you can use some DVD authoring software to load in the two DVD-Video titles and either create your own menus or just have it play without menus. The DVD Maker built into Vista will not do this, nor do I know of any freeware for DVD authoring. Nero 7/8 Ultra will do this, for one. DVD-Video was not designed for combining two discs onto one.

Neither Windows Movie Maker, nor Windows DVD Maker, can import a DVD-Video file structure with the information from the .ifo and .vob files. These files have information about the video structure, menus, playback order, chapters, and more.

Are these films?