One of two microtubules present in the axonemal central pair. It is distinguishable from the C1 axonemal microtubule (also called C1 tubule) by the presence of differing protein components of the projections. [ GOC:cilia http://www.ncbi.nlm.nih.gov/pubmed/9295136 http://www.ncbi.nlm.nih.gov/pubmed/21586547 ]