Double note, double line (tail)

See http://musescore.org/en/handbook/voices, use voice one for Soprano in treble clef and Tenor in bass clef and voice 2 for Alto in treble and Bass in bass clef.
If say, Soprano and Alto have the same note at certain places, just enter both, in voice 1 and 2, so you end up with one note head (actually 2, but one hides the other) with 2 stems, up and down.
As soon as more than one voice is present in a measure, voice 1 and 3 default to stem up, voice 2 and 4 to stem down, when there is only one voice (voice 1), it depends in the position of the note in the staff whether stem is up or down.