the head of the FAA said a second controller on duty woould have been watching the radar scope and not looking out the window...but wouldn't this have allowed the LOCAL (tower) controller to look out the window ?
No. As already has been said, the controller was not watching the radar scope at the time the accident happened. So it's guess work if with a second controller workload would have shifted in a way that the look out of the window at the crucial time would have been made or not. As Scurvy already pointed out,
How many ATC’s do you need??? …. How do you limit their duties to enable them to look for the inconceivable all of the time ?? …. It is not possible ….
So a second controller improves things, but doesn't give guarantees. Maybe look at it this way: with 10 controllers in the tower, chances are very good one would have caught it. Put 10 controllers in every tower all around the globe and fewer accidents will happen. Now you can decide how much money you are willing to pay (scale from 1 to 10) to get your desired aerodrome safety. FAA tends to the number of 2 at the moment (for areodromes of this size) ...after trying for some time with the number of 1.