Mica would have to respond as to whether each 'half' of the P-Activator is stacked; my own guess would be that it follows the classic Fender recipe that the two pieces are humbucking (or hum-cancelling as ALEMBIC prefers) to each other.
I've owned/own ALEMBICs with AXY's and FatBoys, as well as a five-string with the P & J Activators, and while the P is certainly Alembic-ish in tone, it doesn't put me in mind of the other two. Rather, it's just the punchiest, loudest P-pickup I ever heard. Mica says it's the loudest pickup they make, and I believe her.
But it really doesn't put me in mind of the wide shell pickups: My guess would be it's seeing a shorter length of string, or a different image of the string magnetically.
J o e y