Find a song with more than once dance part (duet or more), get a second person to play with, and when picking the song you'll see a prompt "
turn on co-op mode".
Co-op mode is a bit different in that there's a single score for all players, instead of separate scoring.
Once that's on, just finish the song and the achievement will pop. Doesn't matter how well you do (my daughter & I scored I think 3 stars on the song when the achievement popped for me).