We report on calculated outer-shell photodetachment cross sections from the metastable $[\text{core}]nsn{p}^{2}\phantom{\rule{0.2em}{0ex}}^{4}P^{e}$ states in ${\mathrm{Mg}}^{\ensuremath{-}}$ $(n=3)$ and ${\mathrm{Ca}}^{\ensuremath{-}}$ $(n=4)$ negative ions in the photon energy range $\ensuremath{\omega}=0--10\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Double-ionization thresholds for $\mathrm{Mg}\phantom{\rule{0.2em}{0ex}}II$ and $\mathrm{Ca}\phantom{\rule{0.2em}{0ex}}II$ as well as doubly excited thresholds of the residual atoms $\mathrm{Mg}\phantom{\rule{0.2em}{0ex}}I$ and $\mathrm{Ca}\phantom{\rule{0.2em}{0ex}}I$ are located within the photon energy range considered and thus doubly and triply excited states of both ${\mathrm{Mg}}^{\ensuremath{-}}$ and ${\mathrm{Ca}}^{\ensuremath{-}}$ are reached. We use a complex scaled configuration interaction (CSCI) method for the three active electrons supplemented with a sophisticated model potential as developed by Laughlin [Phys. Scr. 45, 238 (1992)] to account for core-valence interactions. The CI calculations are based on ${\mathrm{Mg}}^{+}$ and ${\mathrm{Ca}}^{+}$ one-electron orbitals expanded in terms of $B$-spline basis set. We compare our cross sections with the only calculation available up to a photon energy $0.25\phantom{\rule{0.3em}{0ex}}\mathrm{Ry}$ $(\ensuremath{\omega}\ensuremath{\sim}3.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ [Zeng et al., Phys. Rev. A 62, 022713 (2000)] performed by using the $R$-matrix method, in which only one $^{4}P^{o}$ resonance for ${\mathrm{Mg}}^{\ensuremath{-}}$ and two ($^{4}P^{o}$ and $^{4}D^{o}$) for ${\mathrm{Ca}}^{\ensuremath{-}}$ were predicted. In this work, by analyzing both the cross sections and the locations of the $\mathcal{S}$-matrix poles in the complex plane, we are able to predict six $^{4}P^{o}$, four $^{4}D^{o}$, and four $^{4}S^{o}$ resonant states for ${\mathrm{Mg}}^{\ensuremath{-}}$, and three $^{4}P^{o}$ and two $^{4}D^{o}$ resonant states in ${\mathrm{Ca}}^{\ensuremath{-}}$.