How do you teach sight reading two hands or multiple voices?

I'm a guitar teacher (horror of horrors!) and a builder of an app for sight reading. One of my users is a piano teacher and she'd really like the option of two hands but I'm unsure the best approach to supporting it. Like most software, my dilemma is making it flexible but also easy to use. What I'm really interested in is how you teach two or more voices especially in the context of sight reading. Here are some ideas:

• incorporate it into the existing difficulty progression (which currently sets rhythmic difficulty and note range, ledger lines etc.)
• Have a new voice difficulty knob: number of voices: one, two (one in each hand), two in right, two in left, three (one in left, two in right), etc... I might be overcomplicating this.

I'm really interested in how experienced piano teachers approach it especially given the different pedagogies such as Conservatory, Suzuki, Faber etc.

Thanks for any insight