I guess it will depend what program you use, but with DVD Shrink you need to use "Reauthor" mode. From here you just drag the two titles into the DVD project window & put them into the order you wish them to play.
When you play them, they will play in that order as separate titles, not chapters. You can also access separate titles with a button on your remote (will vary from player to player).
With CloneDVD, you just need to have the titles you want checked & the "Preserve Menu" option unchecked. You then just click next to the Title no. of the one you want to play by default & a green, right-pointing triangle should appear (if not already).
Unfortunately, it will not automatically play them both when you put the disc in your player - according to @Olli, the only way to access titles beyond the default one is to use the remote.
Hope this helps.