In piano scores, it is common to use both staves (bass and treble clef) to write a musical phrase.

This can be entered in MuseScore as follows:

Enter all notes in one staff:

Ctrl+Shift⇑+↓ moves the selected note or chord to the next staff (Mac: ⌘+Shift⇑+↓.)

See also: Bar line for cross-staff bar lines (i.e. grand staff).