In my opinion if they made a video clip of a song together it would work and be mighty as well..
But if they sang together, umm I don't think it would be even good, the tone of Justin's voice is way different of Selena's, and matching the two voices together is kind of unsuitable, but I hope I'm wrong as they are the cutest couple I have seen yet. :)
I think it would be a good idea cause they are both awesome singers !! But it wouldnt be a good idea cause in the future if they break up and hear that song it will be awkard,... So yeah :)