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 the notation in one staff to begin with. e.g.
Select a note and press Ctrl+Shift+↓/↑ (Mac: Cmd+Shift+↓/↑). This moves all the notes in that voice down/up to the other staff (of the same instrument, it won't work to staves of other instruments), e.g.
Note: If you only want to move certain notes in a chord you need to ensure that they are in a separate voice.
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 height: