So I went to go see the head of the greybeards. He wanted me to learn the fire breathing shout. I ran up to the wall, learned the word and impatiently unlocked the shout. As I was thinking to myself "that's kinda silly... What if I didn't have a soul to spend?" the guy gave him his learning of the word (or spirit, whatever). Curious, I checked my shouts... I had just learned fire breath and spent a soul to unlock it. After he gave me his learning of the word, I'd unlocked and learned the second word!
So, two shouts for the price of one! Not sure how it'll work out with the other 2 fire breath walls, but for now, I have the two words unlocked with no known issues. Dunno if this was a known glitch or not, but I figured some of you might like to hear about it.