my first idea would be to search through win.ini or system.ini. Somewhere in there ought to be a line, that says something like
N being a number that depends on how much other MIDI interfaces (AWE, GUS?) you already loaded.
You should try and copy that line, so that two instances of that driver are loaded, for example:
If that doesn't work because of the filenames being the same, you can try and copy one of the files to mpu401_2.drv or something.
You'll have to find out where the drivers keep their I/O port and IRQ settings. That will most possibly be stored in some INI file, I guess. So you'll have to figure out a way to configure both driver instances separately from each other. If the INI filename is hardcoded in mpu401.drv, you can get a hex editor and modify it. :-)
Those would be my approaches. I hope they help.