Alvin, There's no problem using the center one way after the flat spot is made for the set screw. Just make the flat spot small, and only where the pully set screw will contact the shaft. Remember that when the 2 aluminum pullys are placed on the shaft the set screws should be on the opposite ends of the shaft (facing out). You will see what I mean if you try it, actually you cant put them on with the set screws close together, or the belts will not line up.
The actual one way bearing on the center one way pully is in the middle of the pully itself, and rides on the shaft 3-5 mm away from where the set screw would be. It makes switching from 4wd one way to 4wd full time a breeze cause you can simply slide the shaft out a little at the track and change the pully in under a minute!