If you try to add a stemmed-up note to a stemmed-down note as a chord member, NWC will not let you...
I believe that this is only true if the stem-up note is lower than the stem-down note. Given the logic which NWC used to postion the notes this seem perfectly reasonable to me (but then, I've been using the program for a while).
To get the effect you wish you will have to used layered staves, with the stem-up note shifted to the right, using "extra note spacing".
Yeah, I forgot to tell that it only applies to notes with overlapping stems.