My guess would be that the formats(bitrate/frame rate/ audio codec/res) on the 2 parts are different. Thus Tmpgenc encounters an error as while de-muxing the engine expects the same chars on both parts(full file). So what would be better is to split the file in another demuxer(which ignores the stream params) or to cut the file in the splice point(where Tmpgenc encounters the error( which is usually what I do in Mpeg2vcr(by finding out the error point in Tmpgenc and cutting the offensive/erroneous [stream errors could cause the same prob] part in Mpeg2VCr and generating 2 seperate clips) Then check the 2 Clips with Gspot/AviCodec. If Identical Params the just join again in Mpeg2Vcr(womble.com) or if different params then recode to an identical output format.
Hope this helps. Works for me everytime.