Recent success in beam-driven plasma wakefiled acceleration scheme with two proton bunches propagating through a hollow plasma channel* stimulates the research activity in this area. In this report, we investigate possibilities for additional tuning the structure of the accelerating field by the external magnetic field applied. The structure of surface waves at the channel boundary is of interest, and special attention is paid to the field characteristics that are essential for the wakefield acceleration method (amplitude of the accelerating field, the structure of the deflecting field) and the possibilities of controlling these characteristics by means of the external field.