i am not entirely set on this, but i was thinking one or both of them was an ex-disney star as a kid and were never completely able to make the transition into being taken seriously as an adult. the other one floated on the b/c list for years until they had one massive hit that let them skate along a little better. their management realized both of them would look better as a cutesy couple who was super playful and they go along with it. they're friends, but it's all for show and they have lots of NDA's in their personal lives so they can do as they please in private. it's more of a contract/arranged deal more than anything and they are both fame hungry enough to go with it and don't want to go back to being has-beens or nobodies.