exporting: combine a couple of parts

You can generate a part that combines certain parts of the main score, and then export that 'combo' part
Or you can make parts invisible and then export

See:
https://musescore.org/en/handbook/3/parts#setup-parts
to create specific parts from the score (instead of creating all parts).

See:
https://musescore.org/en/handbook/3/parts#Add_instruments_to_a_part
to create a part having more than 1 instrument.

Also:
https://musescore.org/en/handbook/3/parts#export