How to combine two staves?

I have a bass and treble staff I'd like to combine into one. How do I do that?

I tried following these howto (https://musescore.org/en/node/12345) but the instructions don't seem to work in Musescore 4. I can copy all the bass notes, but there's no way to select just one voice in the treble staff to copy them over into. Anything I do selects everything in the treble staff, so when I paste, the bass notes overwrite everything.