MKVToolNix is a set of tools to create, alter and inspect Matroska files under Linux, other Unices and Windows. They do for Matroska what the OGMtools do for the OGM format and then some.
Press the Add button and first load your .mkv file. Press Add once more and load your .srt subtitles file
Click Add Button in MKV Tool Nix and add the movie to which you want to add the dual audio.
Once added, you will be able to see Audio and Video seperated in the 'Tracks,chapters,tags:' container and all boxes checked for an existing movie.
Now, add the mp3 audio to the container which you wanted as second audio for the movie and it will appear as the next entry in the 'Tracks,chapters,tags:' container.
Now all you have to do is click 'Start Muxing'. Done