I know this is quite an old post now. I have just gone through this. I was using a Vibra 16 and a splitter cable for my SC-7 and the joystick didn't work whenever the SC-7 was connected. I also tried with a Media vision Jazz 16 and that didn't work either. In the end I looked for a soundcard that allowed disabling the joystick (I got an Aztech MMPro16) and bought a separate joystick adapter card, disabled the joystick on the soundcard (but enabled MPU-401) and now works perfectly.

Try finding a Gravis MIDI adapter. They allow you to connect both the joystick and a synthesizer to one MIDI-port.

I've verified on my 386 that Wing Commander (1990 release) works with SoftMPU loaded and a Gravis MIDI adapter connected to the Soundblaster 2.0 with a Gravis Joystick and Roland RA-50 on the other end 😀.

The DE15 connector was originally intended for interfacing the gameport only (in a PC).

Electrically and logically, the MPU-401 and the gameport are two separate devices.

They merely share that mechanical connector.

Which means, that gameport spiltter cables likely do not care about the MIDI part (two pins) at all.

Simplest would be to mod an existing joystick or joystick cable. Or the sound card :

Someone could just attach two pins (plus one for ground if the soundcard gets modded) and redirect them to a separate DE15 plug which connects with the midi cable.

To do so, someone could re-use an old gameport bracket intended for a motherboard, a cheap soundcard, a multi i/o card , for example.