I am not a powder expert, so I may be wrong. I ultimately defer to experts. Here is my understanding:
Powders with different grain lengths will burn slightly different. This is due to the different surface area (think split logs vs whole logs). The H4831SC is formulated slightly different from the "standard" cut to account for this difference (and thus make them equal in terms of burn rate).
If you mix them, I would guess they would average out??? -BUT- I personally never guess on anything related to reloading.... I am creating a controlled explosion 6 inches from my face/eyes..... Not worth the convenience of consolidating powders IMHO....