Merge voices in unison while another instrument is playing

Look at menu option Format > Style… > Score > Hide empty staves within systems
Handbook: musescore.org/en/handbook/4/showing-staves-only-where-needed

To merge two unison voices, you can try menu option Tools > Implode
Handbook: musescore.org/en/handbook/4/implode-and-explode

But for copying a unison section, I generally use the Selection Filter (F6) .
Handbook: musescore.org/en/handbook/4/selecting-elements#exclude-range
You need the target staff to have rests available in Voice 2. Then you can swap the notes in the source staff into Voice 2 (Tools > Voices > Exchange voice 1-2) before you set the Selection Filter to only “Voice 2”. Then cut-and-paste the Voice 2 notes from source staff to target staff, and finally remember to reset the Selection Filter to “All”. The Selection Filter helps to avoid copying across unwanted duplicate info like lyrics, dynamics and staff text.

Another technique is to use a separate hidden staff which is never displayed on the screen or a PDF.