stem direction single voice

Follow on from my previous email - the user's guide says "If an upper and lower voice note share the same pitch and attack location, then Composer will automatically share the same notehead for the two voices, provided that there would be no ambiguity in the interpretation of the duration of either the upper or lower voice note." So the shared note should have both an upward and a downward stem - important for choral singing, but I can't get both stems to show.