How do I add 2 (of the same) instrument(s) onto one stave?

See https://musescore.org/en/handbook/4/working-multiple-voices

MuseScore supports four voices, so you can write notes on a stave for up to four of the same instruments and each voice may have a different rhythm.

If you have the identical rhythm for both voices, you can write them like a chord using the same voice. For example, on a keyboard, write the lowest note first, say a C, and then Shift+G for the G in the same beat above it.