If you are going to actually run a second run of 8 guage through your vehicle then you are totally wasting your time. Nothing will be harmed, of course, but nothing will be benefited. If it is allready ran through the vehicle, and all you have to do is connect it, then go ahead...
Also, if you add a second run of power wire then be sure to add a second run to the ground as well.
But as I said, you will gain nothing at all. I would spend my time upgrading to 4 guage or larger, for when you gain some sense and decide to upgrade amps.