Combining two staves on one printed part in Musescore 4

See https://musescore.org/en/handbook/4/implode-and-explode#Implode

You need to create a custom part that has the two clarinets parts

Click on the parts button at the top of the window and then on the Create New Part button in the top right of the dialogue

You will see a new entry called "Part". Click on the three dots to the right of this name and select "Rename". You can call it Clarinets 1 and 2 or whatever, but do this now as that is the name that will show up in the part and it saves changing that later on.
Select and open the part you have just renamed. It will be blank.
In the instrument panel on the left of the window (use f7 if it is not already visible) open the "eyes" of the clarinet parts you want to show in the part.