I learned to split-grade print from Les McLean, who taught me to test for the soft exposure first, selecting the time that just starts to put tone in the important highlight areas. Then determine the exposure needed for he shadows.
My own observations lead me to believe that the soft exposure brings up the highlights, some of the midtones, and a little of the shadows. The hard exposure affects highlight almost not at all, the midtones to a slight degree, and the shadows to a great degree. Exposing for the shadows first then laying down the highlight exposure will darken the shadows more than what was initially intended. And reduce contrast.
I like to think of split grade printing as an elastic process. You "pin" the highlights where you want them with the soft exposure, then "stretch" the shadows to wherever you want them with the hard exposure.