Hi Matt,
Can you be specific about what requests you're trying to make, and what you're seeing come back?
You only need your one Echo Nest API key for calling the Echo Nest API, regardless what information you pull back (so you can get LyricFind IDs for things, for example). If you're trying to hit LyricFind's API (to actually pull back the lyrics for a LyricFind ID, for example), then yes, you presumably need to get a LyricFind API key. I don't know how you sign up for one, and I think they may not have a free version -- may be commercial only. But I expect you can find more information at their web site.
-Eli