You would have to compleatly re-author such a project. You could physically burn two sessions on a DL dvd, but instead, why not just get a two-sided Single Layer DVD and burn one moovie on each side?
If you shove both moovies together, well for one thing the VOB and the INF files will be all confused and neither moovie will play.
You can try to make new INF files buy downloading INFedit - Copy the Main VOB files of each moovie to a single folder, then drag then into INFedit. It will read the VOB files and generate a new set of INF and BUP files.
Then, burn it into a DL DVD in a folder called VIDEO_TS and see if it runs, it ought to paly one moovie, then the next, if you do it right.
This is actualy interesting, maybe I'll try it. But you have to make sure you do not have any menus in the VOB files. Just the moovies, maybe even some extra audio tracks. A lort of people (and this is what pisses me off when they use DVD shrink) do NOT use 5.1 surround sound by default, and it just makes a Stereo Moovie AC3 format.
If yer using DVD Shrink, always set your audio for 5.1 even if the source is stereo.