Someone with better knowledge of data structures could probably give better advice, but personally, I'd use 2 dictionaries for ease of use. You could do the same with 1 dictionary but access time would increase.
Edit: crap, I was just in the process of writing up some code how I would do it and I saw that Falaina posted this which is the same idea that I was doing only much better: Getting key of value of a generic Dictionary?
