Stile and rail bits — sometimes referred to as “cope and stick” bits — most commonly come as a set of two bits that facilitate the creation of truly accurate frame-and-panel joinery. Because cut profiles on stiles and rails must match perfectly, it’s not surprising that the bits are mirror images of one another. Some “sets” are really single bits that stack one arrangement of cutting edges above the other, making for a fairly tall bit.
In use, you would first make all of your stiles (or rails). Then remove the bit and install its mate in the table, using one of the finished workpieces as a guide to get the height set correctly for machining the mating rails (or stiles). Using a single-bit stile-and-rail cutter is the same process, but there’s no need to remove the bit. When finished with your first set of parts, raise or lower the bit as appropriate so the new cutters come into contact with the workpieces, then cut the matching profiles.
Getting these bits set up accurately can be a trial-and-error process, so be sure to make some expendable practice workpieces to get the setup just right before routing your real workpieces. Once you have the bit set to your liking, cut off and save a short segment of that practice workpiece; it’ll make setting up and repeating the cut that much faster in the future.
Stile-and-rail bits must always be used in a router table. These heavy bits have a lot of metal and a lot of cutting surfaces and angles, making them difficult to control in handheld routing. Restricting their use to the router table is not only much safer, but the rigidity of the table-and-fence setup also makes for better control and more accurate routing.