The second piece brings the two sides together and makes them flow - as a previous posted mentioned, the two sides of the tank are not as disjointed with the second piece added in.
Just speaking from a visual perspective, is there something you can add in the back left corner to fill that in / something high up? Your tank looks great but the right side is heavier with all of the stones. Or maybe the stone wall in front of your original drift wood could build up another row higher towards the (left) wall of the tank (I don't know what the rules are of keeping those fish or how hard those are to stack though!) / taper out or down towards the middle?