Divisi problem

I’m using the “divisi” feature to divide a section into “solo” and “gli altri,” but the solo line uses the sound for a whole section playing, rather than the solo sound. Is there a way to fix this so that the soloist will use the solo sound, while the section uses the section sound? I use NotePerformer for playback.

Unfortunately at the moment there’s no separate control over the different staves in a divisi.