You're not giving too much info, like what format the file is, if you're using Ulead I assume it's an mpeg (?). Did you get it from someplace else already in two parts?
I prefer to use womble to cut and join mpegs, but it sounds to me like the person who split the file didn't do a good job of it. You will probably need to cut off a couple of seconds from the second half before joining it.
Here is some info on Womble if you want to try it, unfortunately it's not free but I think there is a free trial version.
http://www.rita.LT/video_tools/help/splitmpg.htm You can also use TMPGEnc
http://www.rita.LT/video_tools/help/tmp ... itjoin.htm
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