Re: SPH-90-1 with PRS Mc Carty
First consideration - pickup routs. On some older PRS guitars, the pickup routs were only cut to accept open bobbin humbuckers. The sharp corners of a metal cover can foul the rounded corners of the cavity.
Second consideration - Single conductor + braided shield cable. Without modification, you can only connect the Phat Cat pickups in parallel with each other.
In any event, two P90s in series, in phase, usually results in a loud muddy sound.
My pickup choice would be the P-Rails, probably in Triple Shot mounting surrounds. Phase reversal and series/parallel interconnection of the two pickups could be controlled via push/pull volume and tone pots.