how to structure my application?

I am struggling with the following architectural problem:

My app right now has 2 singular resources: user and user_session
(authlogic-based), with appropriate controllers. I have a landing page
that has on it both the "Sign in" form and the "Create new user/Sign
up" form. Both forms point to the :create action of the corresponding
resource controller. The landing page is served by LandingController
with a :index action.

I am not sure what is the appropriate way to implement the :create
actions. Specifically, I am not sure what to do in case of an error:
ideally, I would like to render the landing page with errors, but the
default is to render the :new action, which I don't have, and I could
implement them, but I am aiming to create just one page. Is what I am
doing completely counter to the way Rails expects these things to be
done?

Right now, the UserController.create is:

def create
    @user = User.new(params[:user])
    respond_to do |format|
      if @user.save!
        format.html { redirect_to(@user) }
      else
        format.html { render :action => "new" }
      end
    end
end

The UserSessionController is:
  def create
    @user_session = UserSession.new(params[:user_session])
    if @user_session.save
      redirect_to_stored_or_default
    else
      render :new
    end
  end

One form is to create a new user, the other is to create a new user
session. The two forms should post to their respective create actions.
In the error case, you could still instantiate a blank user session
and a blank user so that when you render :template => "users/new" the
template doesn't barf on not having all its variables ready.