A surgical fastening instrument can comprise a handle and an end effector, wherein the end effector can comprise a first jaw comprising a first cartridge attachment portion and a second jaw comprising a second cartridge attachment portion, and wherein one of the first jaw and the second jaw is movable relative to the other of the first jaw and the second jaw. The surgical fastening instrument can further comprise a first cartridge that is insertable into the first jaw and attachable to the first cartridge attachment portion and, in addition, a second cartridge that is insertable into the second jaw and attachable to the second cartridge attachment portion. In various embodiments, the first cartridge can comprise first fastener portions which are engageable with second fastener portions in the second cartridge.