If you want to keep the original chapters, things get more difficult (though it's probably still easy, I just havent looked into the file format of IFO/BUP)
DVDflick (freeware) has the option for stream copy (no compression/transcoding) so you could add the vob files (just rename them to .mpg first)
DVDflick will let you set chapters every x minutes or on every new video source (file)
It might be as simple as using a hex editor to change chapter times to match the original, I don't know - never done it.
This is probably not your best(easiest) solution, but if no one else has any 'all-in-one' programs, this should at least get your two movies into one with chapters at set intervals.