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.