Swift extension only when conforming to Class AND protocol

I have two protocols with extensions LoginPresenting and LoginDismissing. I want LoginPresenting extension only to apply to UIViewControllers that also implement LoginDismissing. I am trying to do it like so but have so far been unable to work out the syntax.

protocol LoginDismissing : class {
    func loginHasCompleted(withController: UIViewController)
}

extension LoginDismissing where Self:UIViewController {
    func loginHasCompleted(withController controller:UIViewController) {
        //...code ommited
    }
}

protocol LoginPresenting : class {
    func presentLogin()
}

The following is bad code, but I think it explains how I am attempting to make LoginPresenting only apply to UIViewControllers that also implement LoginDismissing.

extension LoginPresenting where Self:UIViewController, LoginDismissing //Syntax error here

    func presentLogin() {
        let lc = LoginViewController()
        let nav = UINavigationController(rootViewController: lc)
        nav.modalPresentationStyle = .fullScreen
        lc.loginDismissingDelegate = self //type LoginDismissing
        self.present(nav, animated: true, completion: nil)
    }
}