Alternator wiring

As the diode has two legs should I use them both or just one? I found the MBRP400100CT diode on ebay so I'm going to order it soon.
You need to cut that diode pair in half, to make 2 separate diodes. Bolt them together with the arrows facing each way, as shown in your diagram. They nestle together nicely with the holes aligning. Install them at either end of your bypass wire (in series with the wire), but it needs to be somewhere with a good heat sink. Insulate all exposed wire w/ self-fusing silicone electrical tape.