I got two of the "Become Ethereal" words. I then assigned one Dragon Soul to the "Become Ethereal" shout. Only the first word is activated. I'd like to assign a second soul to Become Ethereal so that I can get the the second level shout, but there is no longer a second word present in "Become Ethereal" when I open the spells menu and then the shouts menu.

Am I doing something wrong or is there a bug?