How to extract two video files from an MKV container

I have a two part video. I placed both files into an MKV container using MKVToolNix and multiplexed and produced the output file. However, the second video does not play at the end of the first. Obviously I have made a rookie error. So:

1. How do I change the files so that the files are linked. There doesn't seem to be a GUI using the app version on a MAC.
2. How do I extract the video files (and subsequent audio) so that I can start over.
3. Other suggestions. Unfortunately I have deleted the two individual videos.

MAC OS, MKVToolnix 29.0.0 and command line operators, and FFMPEG.

Thanks.

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