A low intensity song is best, especially if your dance partner is a newbie. I honestly thought Un Corazon would be the best song since it's quite slow and easy but it turns out Chismosita was the better option. Since its faster it has more moves but yet seems more forgiving.
Try both anyways. Just my dance partner and I got Chismosita on our first try and we attempted Un Corazon five or six times only getting 9/10 stars each time.
Make sure the first player is recognised as your profile in the pre-song lobby and that you're not showing up as a guest. The second player does not need to be signed in.