As fas as I know, midi is required to have seperate tracks for each instrument.
Wrong. There is absolutely no limitation concerning the usage of GM tracks
in a MIDI file, you can put as many or as few instruments in one track as you want. What you probably are referring to are MIDI channels
, which can indeed only play one instrument at once. But how instruments are assigned to MIDI channels is completely up to the user.
OpenMPT creates a GM track for each mod channel, and that is probably not going to be changed until a full re-write of the MIDI export is going to happen, because I really don't want to dig into that code.