I have to say that PSS and PS2 are pretty different tyres in terms of grip and handling behaviour so changing all four should be the ideal thing to start with. If I would be forced to change just 2 in my car, then I would put PSS at the rear since PS2 on the front are not that bad, at least on dry weather conditions. On the other hand, traction capacity is vastly different between the two. As a result, your plan of PS2 rear and PSS front does not sound very well to my ears.
"The mark of a great car is one whose overall competence exceeds what you should expect from its individual components and the 1M does just that", Chris Harris.
BMW 1M-SOLD-: TECH: Evolve Race+N55mids, Evolve IC, Michelin PSS, ER cp, aFe filter, CDVx, Vorshlag camber plates, BMS OCC EXTERIOR: trunk spoiler, blacklines, black grills, IND goodies INTERIOR: Alcantara steering wheel, steel pedals, custom mats, MPower e-brake.