if it ain't laying in the dirt, then it won't rot. It can get wet but because it is in the air it will dry out.

If you can support the new board, then just add the board and make a double 2x8.  If you cannot support the board, screw the second 2x8 on the bottom of the existing 2x8 and make a "strong back" (flange). Extend the new 2x8 across the span of the existing 2x8 and terminate it at the support.  Only the existing 2x8 would need to rest on the support.  The reason being that the existing board will warp/buckle/deflect/etc. before it will fail and you are making it stronger with the second board (as a flange) than you would be by increasing the web by doubling the board.   Thus, the second board makes the first board stronger.  In either case lots of 2-1/2" screws.  If you are worried about rot, go to a lumber yard and order a double treated board.  It has twice the preservative as what you buy off the rack at Home Depot.  The environmentalists have the amount of preservative regulated to a minimum, and the boards do rot over time.  They warn you about ground contact because of the limited amount of preservative in the wood.  

i'm unclear what "add" means in context ... this 2x8 is where and does what?