The double stemmed notes are primarily for Choirs, Quartets and can apply to some wind instruments.
I have performed in Choirs most of my life, and while I don't have a whole lot of music theory kicking around in my head, I can tell you from experience that when we come across those double stemmed notes, say the C one octave above middle in the treble clef: Both Alto and Soprano will sing that particular note.
In Choir or quartet singing, the stem can be used to indicate which "voice member" of the group sings that particular note. By doing this, you can have the altos sing a note that is higher than the sopranos and obtain a slightly different sound than if the sopranos sang all the higher notes and altos all the lower notes. Doing this however has extreme limitations and for obvious range limitations can not be done with every note combination.
Take care.
Scott
Scott--
While the double stemmed notes are used for the applications that you mention, I think that their use in instrumental polyphonic music--Bach, for example--is not less important.
