Two mirrors would muck up the story. Only one of them will get it and lets be honest here, it will be the Alliance. It's been something that a lot of alliance players have wanted for some time and with Danath having his little mention in Cata it's obvious he wants to come back in force and reclaim it.
Be prepared for whining if the Alliance don't get it is all I'm saying, and I don't think it will be entirely unjustified either.
The other issue with Alliance vs Forsaken is that it would break the truce that appears to have been put in place due to the rebellion. Varian has gona ahead and told Vol'jin that the Alliance don't want to be their enemies. Vol'jin's dialogue also indicates he's got contact with her. So for her to go attack the Alliance would break this trust between the Rebels and the Alliance.
On top of this it would remove any scape goating Sylvanas could possibly accomplish if she attacked the Alliance at this point. So far she's gone underground because until then it's easy for her to scapegoat her attacks (but not methods ultimately) on Garrosh.
5.3 has also given us a president that it's ok for one side to have more scenarios than the other.
Edit: I'm already fired up by the notion that people can even suggest this won't be an Alliance pov scenario
, or at least that Danath won't reclaim it in canon.