  A theoretical study the all two-photon transitions from initial bound states
with ni = 2, 3 in hydrogenic ions is presented. High-precision values of
relativistic decay rates for ions with nuclear charge in the range 1 =< Z =< 92
are obtained through the use of finite basis sets for the Dirac equation
constructed from B-splines. We also report the spectral (energy) distributions
of several resonant transitions, which exhibit interesting structures, such as
zeroes in the emission spectrum, indicating that two-photon emission is
strongly suppressed at certain frequencies. We compare two different approaches
(the Line Profile Approach (LPA) and the QED approach based on the analysis of
the relativistic two-loop self energy (TLA)) to regularize the resonant
contribution to the decay rate. Predictions for the pure two-photon
contributions obtained in these approaches are found to be in a good numerical
agreement.
