You can use either perfectly well.

Make trial prints with max magenta to get the blacks, then make a max magenta with good blacks and overlay with trials of max yellow.

With the VC head, just use 5 for black and zero for white.

This whole split thing is a big waste of time and paper except for very specific prints like where you need to burn in highlights. For grade 2/3/4 you get the very same result with without split.

You can also print with normal contrast and do an overburn with #5 or 150 magenta to get better blacks.

Save yourself a lot of time.