  We investigate the process of nuclear excitation via a two-photon electron
transition (NETP) for the case of the doubly charged thorium. The theory of the
NETP process has been devised originally for heavy helium-like ions. In this
work, we study this process in the nuclear clock isotope $^{229}$Th in the $2+$
charge state. For this purpose, we employ a combination of configuration
interaction and many-body perturbation theory to calculate the probability of
NETP in resonance approximation. The experimental scenario we propose for the
excitation of the low lying isomeric state in $^{229}$Th is a circular process
starting with a two-step pumping stage followed by NETP. The ideal intermediate
steps in this process depend on the supposed energy $\hbar\omega_N$ of the
nuclear isomeric state. For each of these energies, the best initial state for
NETP is calculated. Special focus is put on the most recent experimental
results for $\hbar\omega_N$.
