Totally esoteric side thought-
I use an Arturia Beatstep.. and they send midi out on a single, MONO 1/8″ plug.
Then there is a little dongle-cord adapter that converts it to 5-pin.
This thing: 
Maybe this is a known standard? It seems like you could send bilateral midi messages on a stereo 1/8″ this way.. but I have totally not looked into it.
