a nice little addition to the existing “Colour voices out of range”, etc. would be to colour all notes on different staves that have the exact same sounding pitch.
I don’t know if it is trivial to implement (such things may sound easier than they are).
In any case it would be neat for arranging.
Do you mean including register (ie colours not only for notes A-G but also based on octave)? If not, you could temporarily change the default notehead set to Figurenotes, which has different colours for different pitches.
In this specific case (arranging for two pianos) the register should be included, so you can check for duplicate notes, so you can decide that the one or the player should play that note.