There are a number of options described in this thread. If you are willing to try a command-line FFmpeg method, it will give you best results for two reasons: 1) It operates on final exported video rather than stitching MLT files together on a common timeline, which eliminates a lot of fragility and makes each section fully independent for further editing; and 2) the stitched video is truly stitched, and not re-encoded which would cause a generation of quality loss. Obviously, all videos would need to be exported with the exact same settings for this to work.