There are proposals to utilize the energy recovery linac (ERL) as driver for the light sources with ultra-low emittance. The ERLs have co-propagating beams inside the same vacuum vessel. These beams can have different trajectories, which should be distinguished by beam position monitors (BPM). In this paper we present a concept of a BPM utilizing the phase information for calculation individual position of each of the two beams (accelerating and decelerating). The practical realizations are presented and achievable accuracy is estimated.