In the Score Editor, a MIDI track’s staff can be displayed as either a single staff or as a double staff, e.g. for piano (Score Settings>Staff>Polyphonic).
Additionally, just like in the MIDI Key Editor, you can open several MIDI tracks at once in the Score Editor, and they will appear in the same order as in the Project window (allowing orchestral scores, for example).
Create two MIDI tracks (or Instrument tracks)… one for the melody, and the other for the piano. Select the Parts on both MIDI tracks and open them in the Score Editor (They will, initially, each be “single staff”).
Click on the Piano staff, to select it.
Which version of Cubase do you have? Only the full version has the Advanced Score Editor, with its page Mode (for proper printing etc.). The next step depends upon which version you have…
If you don’t have the full version of Cubase, having selected the Piano staff, go to MIDI Menu>Scores>Staff Settings, and change “Staff Mode” from “Single” to “Split”. You now have the Piano grand staff.
If you do have the full version of Cubase, go to Scores Menu>Settings>Staff>Polyphonic, set the Staff Mode to either “Split” or “Polyphonic” (this latter one is more complex, and you really need to consult the manual, to learn how to use that ). Additionally, with the full version of Cubase, you can then click on the Layout tab, and create brackets for the piano’s grand staff.
But, please, do read the manual .