I share Dance Central 2 with my brother / sister / boyfriend / girlfriend but we each want to link our accounts. Can we use the same Account Linking Code?

You cannot use the same Account Linking Code for multiple accounts, but each Gamertag has its own code anyway. Simply navigate to the Account Linking Code screen in Dance Central 2 while signed in to the Gamertag you want to link and you'll get a unique Account Linking Code. If you're sharing a Gamertag to play, then you'll need to share an online account as well.