Problems combining vob files



i downloaded part 1 and part 2 of a dvd. when they were done downloading they were in two different vts video subfolders because again i had to download part 1 and then part 2 when i try to combine them in nero i get the message something along the lines of " a special entry already present on target location original joliet or iso filenames cannot be identical cannot move vts_01_1.vob to new location". when i changed one from vob1 to vob2 it gave me the message dvd-video compliance test failed and then it said in the log video_ts.ifo and video_ts.bup are not present. this is my first time trying to burn a downloaded dvd, i need to figure out how to combine these files please help