If you are viewing this website in Chrome, then you can use it to connect two MIDI keyboards. The previous sentence makes me feel like I’m in the 21st century.

I’m amazed that you can have websites that fully interop with MIDI specs (though not compatiable on all browsers still). I had attempted this kind of thing before using Go + PortMidi but it was always tricky getting it to work on different operating systems. Since Chrome is easy to install everywhere, you can actually have chrome everywhere supporting your MIDI devices, with just a little Javascript code.

You can find the repo for this code here: github.com/schollz/midi2midi.