Record your independent tracks as you like.
Freeze and Combine The Events into a single Track
From the main Project Display…
In the track inspector, or in the Track Pane, set your Right hand channel to something like 1 or 2, and your Left Hand channel to something like 3 or 4.
Hold ctrl and select the two tracks. Right click one of your selected tracks and choose “Select All Events”.
Freeze the two tracks. (Top Menu/MIDI/Freeze MIDI Modifiers)
When you freeze a MIDI track like this it will hard set whatever you have established in the Track Inspector into the MIDI events on the track. This includes the ‘MIDI Channel’ that was set in the Track Inspector. The Score Editor treats the MIDI channel of individual MIDI events as a (Voice).
Toggle the Right Hand Track so it it displays your Track “Lanes”. Drag your Left Hand Element into an empty lane of the Right Hand part. It will snap on top of the Right Hand Part when you let your mouse button go. Drag it again into an empty lane.
If you don’t see this Lane icon, drag the track so it is taller. If you still don’t see it, you can add it to MIDI and/or Instrument tracks with the settings cog in bottom right of the Tracks pane.
By having the MIDI parts/events on the same track, and sorted by channels, you should find that you can sort it all out on a single grand staff, and each note on a different channel will be an independent ‘voice’ (like in contrapuntal music). I believe the Score Editor built into Cubase can handle up to 4 voices per track or stave.
Note, if something is missing or you end up with two grand staffs again, select the track, right click it, and choose ‘select all events’ before opening the score editor. That will force all of the events on the track into the same stave system.
You can delete the now empty Left Hand Track, fold up the lane view, and change the main Playback channel back to what your instrument needs in the Track Inspector.
Note, you could also simply freeze the tracks to different MIDI channels, and truly merge them together, so all the notes are in the same event. See Merge MIDI In Loop, and Bounce MIDI.