Here is a situation for you on high school baseball. I'm not giving the answer that we decided in our association because I want to see what other people think of this one. Runner at first and third, no outs.
Runner takes a lead off of first about half way to second almost in the grass.
He gets the pitchers attention and the pitcher steps off of the mound and starts walking to the runner. The runner doesn't back up any more but starts to get in a run down at the grass line of the infield. Mean while the runner at third is trying to score. This situation is just like a normal run down so that the runner at third can score, except, the runner at first is backed up to the grassline in the infield. How about this opinion on your rundown situation where you have runners on first and third with no outs.