The superposition principle makes this all quite easy — we just add together the waveform files that sf2sound produces for the two voices. That file represents the combined voices, and we use text2sf to produce the audio file. For more voices, simply add more waveform files!
Our work so far is quite primitive, both musically and as a software product. But we are having a lot of fun, and learning a lot. Eventually we hope something polished and elegant will come out of this.