Re-encode with FFmpeg using timestamps and duration in the video denoted as hours:minutes:seconds.milliseconds (hh:mm:ss.ms), -ss is the start position of the encode, -t is the duration, make sure to use lossless codecs again to preserve the quality of video and audio
ffmpeg -ss hh:mm:ss:ms -accurate_seek -t hh:mm:ss:ms -i "title.cd1.mkv" -ss hh:mm:ss:ms -accurate_seek -t hh:mm:ss:ms -i "title.cd2.mkv" -filter_complex "[0:v:0][0:a:0][1:v:0][1:a:0]concat=n=2:v=1:a=1[outv][outa]" -map "[outv]" -map "[outa]" -acodec flac -vcodec ffv1 -level 1 -coder 1 -context 1 -g 1 "title.mkv"