You need two players present on the pre-dance screen to initiate co-op mode. Press
to activate Co-op mode. Then, simply dance to the song. Both dancers will be contributing to the same score. If you're combined score is Superstar, you'll also unlock the following achievement:
For this achievement, Co-op mode just has to be activated. So, even if your partner leaves in the middle of the song because they're tired or something, as long as you completed the song while Co-op mode was active, you'll unlock the achievement.
Repeat this nine more times to unlock the following achievement: