Theory of the multiphoton cascade transitions with two-photon links is considered within two different approaches: Quantum Electrodynamical (QED) and phenomenological Quantum Mechanical (QM). A problem of regularization of the cascade contributions is investigated in detail. It is argued that the correct regularization should include both initial and intermediate level widths in the singular energy denominators. This result follows both from the QED and from the QM approach. Particular transitions $ nl\rightarrow 1s+2\gamma $ with $ nl=3s,\;4s,\;3d,\;4d $ and $ nl\rightarrow 1s+ 3\gamma$ with $ nl=3p,\;4p $ are considered as an examples. The importance of the proper cascade regularization is also demonstrated.