Both Marvel Studios and Fox can use the two characters. Fox Studios cannot mention The Avengers and Marvel Studios cannot mention Magneto or mutants.
Anyone else feel like the Scarlett Witch is going to have a pretty big role? I'm waiting for Joss Whedon to start writing in the strong female lead. It doesn't seem like Black Widow is going to get it, but I could be wrong because she has less star-power to fight in the new Cap. America film.