You can register two letter .cc domains. There ARE a few left, however, I have found it easier to register something with a semi-meaningful number- letter combination. I recently registered 1k.cc and 1i.cc
1k.cc = 1000
1i.cc = 1 eye?
Anyway, I also found a few LL.cc that are still not registered. I just don't know if they are worth it at this point since they are unpopular letter combinations.