I used to use one with a shorter cutter to work the face plate recesses and start the lock body mortise. The other with a much longer pocket hole cutter to finish the lock recess off - it"s a safety thing, really, because the lònger pocket hole cutters extend below the base of the router even when the body is fully raised. If you just use a single cutter you can't get full depth of cut for the lock body and have the ability to recess the face plates.
BTW a corner squaring chisel speeds up the task of finishing the face plate recesses (useful when doing hinge recesses, too)