It may seem like a simple idea to create a set of pairs of tightly coupled midi tracks and audio outs of a synth that supports multiple outs. But it really doesn't work that way down below. There is nothing in the world of VSTi's and tracks that can force the midi on one and only one track to generate audio to one particular output of several.
 
Each instance of a synth has only one midi input port, and it is completely up to the synth to assign incoming midi events to particular audio outputs; the host has no control. A synth could route output by midi channel, by keyboard range, or even by keyswitch or other controllers. There is just no API for the host to control routing of midi to particular outputs of a single instance of a synth.
 
Simple instrument tracks are possible only by the restriction of allowing only one midi track to be assigned to the (lone) midi input port of a synth that is configured with just one audio output.
 
[That said, simple instrument tracks still have some flaws in the way their controls work, and it would be nice if they would be improved.]
 
(Edit: it needed paragraphs)