With 400A load currents, condition of voltage drops and cables ampacity will be better in comparison old situation. Of course it should not forget that we must support whole capacity of power transformers (600A) for future consumers. Also when we use two individual power cables instead one greater cable, we must care of cable shields capacity. Only if the ratio of phase wire cross section in two individual cables is same of ratio of two neutral cross sections, we have not any unbalance flux due to unbalance loads. As you know over unbalance flux can create over eddy current in cable shields.