  We give a detailed account of Agol's theorem and his proof concerning
two-meridional-generator subgroups of hyperbolic 2-bridge link groups, which is
included in the slide of his talk at the Bolyai conference 2001. We also give a
generalization of the theorem to two-parabolic-generator subgroups of
hyperbolic 3-manifold groups, which gives a refinement of a result due to
Boileau-Weidmann.
