The electrostatic instability paused by a tangential discontinuity of velocity in two semi-infinite homogeneous regions of a cold and collisionless plasma subject to a uniform magnetic field is investigated using two fluid equations. A general dispersion relation is derived allowing for interstreaming of charged particles in each region in addition to a slippage of one region over the other. Special cases are then discussed and the criteria for stability, monotonic instability and growing wave instability are derived.