At the time of the Kings ...there were often duets guitar and harpsichord ...because there were already pianos ...they arrived later. So by searching in the MENUETS it is possible to find DUOS in the form of - REPLY - one instrument plays ...and the other one answers him ...Then, it is a little the same thing when the first pianos arrived ...see in GALLICA BNF
And then today, almost all scores are multi-instrument, one plays the melody and then the other, or one does a chorus the other does the rhythm. Does EVERYTHING have to be written in a score? Can't there be some improvisation on a theme?
Good fishing for scores ...