This instructions whall work on any system (Mac OS X, Linux) supporting installation of mkvtoolnix
If you don't have Homebrew installed, do it first, the continue with following command:
brew install mkvtoolnix
mkvmerge -i myFile.mkv
File 'myFile.mkv': container: Matroska
Track ID 1: video (V_MPEG4/ISO/AVC)
Track ID 2: audio (A_DTS)
Track ID 3: subtitles (S_TEXT/UTF8)
Track ID 4: subtitles (S_TEXT/UTF8)
Chapters: 22 entries
Track ID 3 and 4 are subtitles.
mkvextract tracks myFile.mkv 3:myFile1.srt 4:myFile2.srt
Enjoy!