We study binary mixtures of Bose-Einstein condensates confined in a two-well potential within the framework of the Gross-Pitaevskii equation. We re-examine both the single component and the binary mixture cases for such a potential. We investigate the most usual dimensional reductions used to solve the Gross-Pitaevskii equations, including the one proposed by Reatto and collaborators. To this end, we compare numerical simulations of the 1D reductions with the full 3D numerical solutions of the Gross-Pitaevskii equation. Our analysis considers an experimentally feasible binary mixture of an F = 1 spinor condensate where two of its Zeeman manifolds are populated.