Sometimes your singing with your friend(s) and maybe they are not very good at it and it gets a little akward. That made me think of two modes to add .
1. I think it would be nice to have freeplay with notes, so we can see if we are doing it right, but no points.
2. Team play. Both players singing at the same time but the points would be the average of both together like [(player 1 + player 2 points)/2]
(5000 + 7000)/2
would be 6000 points at the end. Get it? Dont show the points separetly.

Great Idea!
Its Freeplay with notes very nice!

Something I thought of while reading your post - is there an option to have the players sing something else? It would be cool to sing a proper duet. It could either be pre-made and included in the song file, or use a setting like each player singing one line.