We present ab initio two-dimensional extended Hubbard-type multiband models for EtMe${}_{3}$Sb[Pd(dmit)${}_{2}$]${}_{2}$ (where dmit is 1,3-dithiole-2-thione-4,5-dithiolate) and $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu(NCS)${}_{2}$ [where BEDT-TTF is bis(ethylenedithio)-tetrathiafulvalene] after a downfolding scheme based on the constrained random-phase approximation (cRPA) and maximally localized Wannier orbitals, together with the dimensional downfolding. In the Pd(dmit)${}_{2}$ salt, the antibonding state of the highest occupied molecular orbital (HOMO) and the bonding/antibonding states of the lowest unoccupied molecular orbital (LUMO) are considered to be the orbital degrees of freedom, while, in the $\ensuremath{\kappa}$-BEDT-TTF salt, the HOMO-antibonding/bonding states are considered. Accordingly, a three-band model for the Pd(dmit)${}_{2}$ salt and a two-band model for the $\ensuremath{\kappa}$-(BEDT-TTF) salt are derived. We derive single-band models for the HOMO-antibonding state for both of the compounds as well. The HOMO antibonding band of the Pd(dmit)${}_{2}$ salt has a triangular structure of the transfers with a one-dimensional anisotropy, in contrast to the nearly equilateral triangular structure predicted in the extended H\"uckel results. The ratio of the larger interchain transfer ${t}_{b}$ to the intrachain transfer ${t}_{a}$ is around ${t}_{b}/{t}_{a}\ensuremath{\sim}0.82$. Our calculated screened onsite interaction $U$ and the largest offsite interaction $V$ are $\ensuremath{\sim}$0.7 and $\ensuremath{\sim}$0.23 eV, respectively, for EtMe${}_{3}$Sb[Pd(dmit)${}_{2}$]${}_{2}$ and $\ensuremath{\sim}$0.8 and $\ensuremath{\sim}$0.2 eV for $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu(NCS)${}_{2}$. These values are large enough compared to transfers $t$ as $\ensuremath{\sim}$55 meV for the Pd(dmit)${}_{2}$ salt and $\ensuremath{\sim}$65 meV for the $\ensuremath{\kappa}$-BEDT-TTF one, and the resulting large correlation strength ($U$$\ensuremath{-}$$V)/t\ensuremath{\sim}10$ indicates that the present compounds are classified as the strongly correlated electron systems. In addition, the validity whether the present multiband model can be reduced to the single-band model for the HOMO-antibonding state, widely accepted in the literature, is discussed. For this purpose, we estimated the order of vertex corrections ignored in the cRPA downfolding to the single-band model, which is given by ${W}^{\ensuremath{'}}/D$, where ${W}^{\ensuremath{'}}$ is a full-screened-interaction matrix element between the HOMO-antibonding and other bands away from the Fermi level (namely, HOMO-bonding or LUMO-bonding/antibonding bands), whereas $D$ is the energy distance between the Fermi level and the bands away from the Fermi level. In the present materials, ${W}^{\ensuremath{'}}/D$ estimated as 0.3--0.5 signals a substantial correction and thus the exchange process between the low-energy HOMO-antibonding and other bands away from the Fermi level may play a key role to the low-energy ground state. This supports that the minimal models to describe the low-energy phenomena of the organic compounds are the multiband models and may not be reduced to the single-band model.