  Spectral collapse, the transition from discrete to continuous spectrum, is a
characteristic in quantum Rabi models. We explore this phenomenon in the
two-photon quantum Rabi model in optical phase space and find that, in the
so-called degenerate qubit regime, the collapse is similar to that happening in
the transition from a quantum harmonic to an inverted quadratic potential with
the free-partical potential as transition point. In this regime, it is possible
to construct Dirac-normalizable eigenfunctions for the model that show well
defined parity. In the general model, we use parity to diagonalize the system
in the qubit basis and numerically find that the qubit frequency does not
change the critical point where spectral collapse occurs.
