Does the ASUS PN51 have the same storage layout as the PN50?

Yes, the ASUS PN51 keeps the same two-drive storage layout as the PN50 — one M.2 2280 PCIe 3.0 x4 slot and one 2.5-inch SATA III bay. ASUS didn’t add external SATA ports, so you get two drives: one fast, one flexible.