A two-piece diaphragm assembly for a fluid powered diaphragm pump is provided. The diaphragm assembly includes a key as part of a overlay diaphragm extending towards a recess in a backup diaphragm, for properly aligning the overlay diaphragm and the backup diaphragm during installation of the diaphragm assembly. The diaphragm assembly also provides an improved mechanism for attaching the inner plate of the diaphragm assembly to the outer plate of the diaphragm assembly. A method of installing a two-piece diaphragm assembly onto a fluid powered diaphragm pump is also provided.