Parts in Yousician Piano are generated based on Rehearsal Marks in the score. To do that in Finale, select the first note of a measure where you would like to start a new part, and in Simple Entry mode, press "X" and then "M". (Alternatively, use the expression tool to enter a Rehearsal Mark manually.)
In Sibelius it's done by pressing "Cmd+R" in Mac OS X and "Ctrl+R" in Windows.
Hint: The text of the rehearsal mark should be the title of the part in Yousician.