Your best bet is to use DVD Shrink. First, use the Re-Author function to take only the movie (leaving credits and menus as those will take a lot of room) of the first disc. Do not shrink this down, just make it into an .iso file. Now, use Daemon Tools to mount the .iso file you just made. Then, go into DVD Shrink again and load both the 2nd disc and the 1st disc (from the virtual drive). Now, just the Re-Author function again, putting both halfs back-to-back. Use the compression needed to shrink it down to size and presto! you got your movie.
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