If I have only one dance pad, can my wife and I take turns playing 'multiplayer', or do we just have to play single player and share? (so we can record our high scores separately for example.)
You cannot "hotseat" one pad with different scores.
You have to have 2 for multiplayer. You can have separate player accounts that will track different scores when you have more than one pad, but you cannot use different accounts to track different scores at the same time with one pad.
You'll have to play single player under one profile and share the score.