In piano scores, it is common to write a musical phrase extending across both staves—bass and treble. 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+↓):
Note : this moves the whole chord, not just a single note from a multi note chord. If you need notes in the old staff at the same place, use voices.
To adjust the beam, double-click it to show the handles. Use the keyboard arrows or drag the handles to change the beam angle and position: