It is surprising that there is no split point set for step-time entry into the piano part, and no way of setting one.
One of the ways I use Sibelius to compose is by playing a passage of music into the piano part as a guide and then orchestrating it. Because the whole thing comes out in one clef, it's difficult to later read what I've played in. I don't want to have to waste time editing this piano part, that would defeat the whole purpose...I might as well make a handsketch and work off of that, like in the old days. And I certainly don't want to waste time and energy playing things in "hands separately".
It seems obvious that if you play a two-handed passage into a piano part it would come out in two staves....even in step time.
My preference would be for a default C4 split point that could be turned off completely or set to a different pitch.