High level ab initio calculations using a complete active space self-consistent. field (CASSCF) and multiconfigurational quasi-degenerate perturbation theory (MCQDPT2) methods as well as density functional theory (DFT)-based calculations with different exchange-correlation energy density functionals have been performed for predicting the relative stability of the proton-transferred vs hemi-bonded isomers of (H2O)(2)(+) and (H2S)(2)(+) species. For (H2O)(2)(+), DFT calculation using conventional exchange-correlation functionals predicts the hemi-bonded structure to be the ground state while use of full or half Hartree-Fock exchange and local correlation predicts a higher stability of the proton-transferred structure in agreement with ab initio results. For the (H2S)(2)(+) system, all of the methods lead to the prediction of lower energy for the hemi-bonded isomer. No regular trend of the exchange-correlation energy component with the total energy difference is however observed. Dynamical electron correlation effect incorporated through MCQDPT2 is found to be much stronger in (H2O)(2)(+) as compared to (H2S)(2)(+). An analysis of the nature of interactions involved in, the (H2O)(2)(+) and (H2S)(2)(+) systems within the framework of Bader's topological theory of atoms in molecules is also presented through the plots of the Laplacian del(2)p of the electron density p(r) and also other related quantities at the bond critical points with the objective of rationalizing the relative stability of the two isomers in both (H2O)(2)(+) (H2S)(2)(+)