In the present study, a numerical simulation of double-diffusive natural convection in a square cavity with varying inner heater positions and temperature-dependent viscosity is reported. The porous cavity is occupied with a binary fluid, and the combined effects of thermal and solutal buoyancy forces are investigated for different control parameters such as the Lewis number (Le), buoyancy ratio (N), thermo-viscosity parameter (m), and block location (left–top, center, right–bottom). The governing equations of mass, momentum, energy, and concentration, using the Darcy–Brinkman–Forchheimer model in vorticity–streamfunction formulation, are solved by employing a validated numerical method. Special focus is placed on the effects of temperature-dependent viscosity on the flow behavior and thermal and solutal transport rates. The simulation results show the flow organizes into two counter-rotating cells, whose intensity strengthens with increasing m, thinning thermal and solutal layers. Heat transfer, quantified by Nu, decreases with Le and, for N = −2, becomes nearly independent of Le for Le ≥ 5. Mass transfer, Sh, rises with Le for both aiding and opposing buoyancies. Heater position controls performance: LT > CC > RB for N = −2 and RB > CC > LT for N = 2.