Ferroelectric tunnel junctions (FTJs) utilizing an in-plane head-to-head ferroelectric domain wall (DW) have recently been realized, showing interesting physics and new functionalities. However, the DW state in these junctions was found to be metastable and not reversible after applying an electric field. In this work, we demonstrate that a stable and reversible head-to-head DW state can be achieved in FTJs by proper engineering of polar interfaces. Using density functional theory (DFT) calculations and phenomenological modeling, we explore the DW stability by varying stoichiometry of the La_{1-x}Sr_{x}O/TiO_{2} interfaces in FTJs with La_{0.5}Sr_{0.5}MnO_{3} electrodes and a ferroelectric BaTiO_{3} tunnel barrier. For, x≤0.4 we find that the DW state becomes a global minimum and the calculated hysteresis loops exhibit three reversible polarization states. For such FTJs, our quantum transport calculations predict the emergence of a DW tunneling electroresistance effect-reversible switching of the tunneling conductance between the highly conductive DW state and two much less conductive uniform polarization states.