I have worked SO hard to get two sims singing skills as high as they can so far, one is 9 Singing, and the other is 8, you would expect those two are capable on putting on a good dual singing show.
Problem is again and again, one sings the other doesn't, it's usually the one on the right, doesn't even open her mouth, the entire song she does nothing but eye contact with the other singer and bobs her head with the beats, but never sings a single note, and keeps her mouth closed the entire time. To put things straight the second singer just looks like a complete idiot when she doesn't sing the next note that was obviously for her.
This wasn't a problem when both had a low singing skill, at least both try to sing together (Even though it sounded bad), but now they are high singers, they can no longer sing correctly together, maybe it's the type of song, I tried a few of the list, but it's not working with the Jazz songs.
This is enough to quit singing in frustation if they are no longer capable of putting in a good show...