Automatic cropping and picture size

[gonzus]

Here is a real use case that I would love to hear opinions on how to handle. Maybe this is basic stuff...

I am converting my From the Earth to the Moon DVDs into Matroska files, so I ripped them with DVDfab. Each DVD has two long "movies" in it, that I converted with Handbrake 0.9.3 into h.264 format within a Matroska container, using exactly the same parameters (including automatic cropping). Then on a whim I thought it would be nice to join the two movies into a single one using mkvmerge. But when I tried the command line mkvmerge -o mixed.mkv part1.mkv +part2.mkv I got an error message saying Error: The track number 1 from the file 'part2.mkv' cannot be appended to the track number 1 from the file 'part1.mkv'. The track parameters do not match. Then I opened both MKV files with mkvmerge GUI and noticed one of them is 631x480 and the oher is 632x480 (the original files are both 720x480). My guess is the automatic cropping is getting these slightly different results in both cases.

What do you advice I should do? How do I determine the cropping settings used for the 632x480 case, so that I can manually force them in the other case? Is this the correct course of action for this situation, or should I approach this some other way?

Thanks and best regards.