Say you have 2 files, file1.vob and file2.vob. To join these two files and create a new file, say file3.vob, you need to type the following in the Terminal:
cat file1.vob file2.vob > file3.vobAnd thats pretty much it. Mind you this process can take some time depending upon the size of your files but usually it shouldnt take more than 10 mins for standard DVDs.