We show the boundary controllability to stationary states of the Stefan problem with two phases and in one dimension in the space variable. In the case of an initial condition that is a stationary state and for a time of control large enough, we also obtain the controllability to stationary states with the sign constraints associated with the problem. Our method is based on the flatness approach that consists in writing the solution and the controls through two outputs and their derivatives. We construct these outputs as Gevrey functions of order σ so that our solution and controls are also in a Gevrey class.