You can't generally directly merge video dvds. You can however rip the vob files from them and reauthor them into a new disk. There are many options for extracting vobs, such as mplayer, dvd::rip, k9copy, vobextract. I usually use the tccat tool from transcode myself.
For reauthoring, I suggest dvdstyler.
Note that commercial dvd's are generally much larger than your average dvd-r, so you may not be able to directly fit everything onto a new disk. You might have to reencode the video to a smaller size and/or bitrate. Vob files are simply mpeg2 video with either ac3 or mp2 audio in a specialized container, so as long as you keep to the dvd video specifications
you should be ok, unless it has subtitles and/or multiple audio tracks, which are not as easy to deal with.
I've never tried it, but you might look into encode2mpeg
, which has a lot of features, including subtitle-handling ability.