If he has already signed to Deadpool, and being that its a Marvel movie, it would probably cost him far too much to get out of that contract. I cant see a Marvel film, where he has already played the character, be too willing to let him go off and do a DC film at the expense of theirs.
Of course, if the filming of one does not coincide with the other, he can still do both, as it wouldnt be the first time an actor has been in a movie for both companies (James Marsden and Halle Berry spring to mind).
