I want to record a duet with the parts completely separated, so that by turning the balance knob you can hear one, the other, or both.

This lets you hear the parts play together, or you can play along with either part alone.

The source files can be midi, wav, or whatever. I'll probably create them with notation software initially.

Any suggestions for how to put them on separate stereo tracks? I'm looking for a software solution, I don't have a mixer.