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⇑+↓.)
If you want to move the beam, double click the beam to show the handles. Drag the handles to adjust the layout.
See also: Bar line for cross-staff bar lines (i.e. grand staff).