How to add more staff in the selected measure?

Enable "Hide empty staves"https://musescore.org/en/handbook/layout-and-formatting#style-menu

Enabling hide empty staves is absolutely the way to limit which staves are shown, but you need to add staves to the existing piano rather than adding a new instrument. When you press i to open the instruments dialog select staff 1 under the piano and click Add Staff (not add linked staff) then do the same for staff 2 and click OK. You may need to fix the treble clef you added to staff 1. You also need to add notes before you hide empty staves or you won't be able to see them to add notes to them.