Krist I though tickets for the Zurich - Munich train now came with a reservation included?
No, it was only in the Summer of 2024 duriing the Euro in Germany because reservation was mandatory. It is difficult to predict the occupancy rate of these trains in advance, during one of my trips 125% (!), but during the last one a few days ago about 25%.
It is impossible to add a second composition, the platforms of the intermediate stations in Germany and perhaps the passing loops are too short.