  We investigate the possible size of two-loop radiative corrections to the
Higgs trilinear coupling $\lambda_{hhh}$ in two types of models with extended
Higgs sectors, namely in a Two-Higgs-Doublet Model (2HDM) and in the Inert
Doublet Model (IDM). We calculate the leading contributions at two loops
arising from the additional (heavy) scalars and the top quark of these theories
in the effective-potential approximation. We include all necessary conversion
shifts in order to obtain expressions both in the $\overline{\text{MS}}$ and
on-shell renormalisation schemes, and in particular, we devise a consistent
"on-shell" prescription for the soft-breaking mass of the 2HDM at the two-loop
level. We illustrate our analytical results with numerical studies of simple
aligned scenarios and show that the two-loop corrections to $\lambda_{hhh}$
remain smaller than their one-loop counterparts, with a typical size being
10-20% of the one-loop corrections, at least while perturbative unitarity
conditions are fulfilled. As a consequence, the existence of a large deviation
of the Higgs trilinear coupling from the prediction in the Standard Model,
which has been discussed in the literature at one loop, is not altered
significantly.
