We simulate two back-to-back full-scale tidal turbines using an in-house computational free-surface flow code. We briefly present the mathematical formulation of the computational framework. We first validate the proposed method on a single turbine configuration. A mesh refinement study is conducted to ensure the result is converged. We then quantify the wake effect and free-surface effect on tidal turbine performance by a case study. To investigate the free-surface effect, we perform both pure hydrodynamics and free-surface simulations. The time history of thrust and production coefficients is quantified. In both pure hydrodynamics and free-surface flow simulations, thrust and production coefficients of the downstream turbines drop significantly due to the velocity deficit in the wake. By comparing the result between free-surface flow and pure hydrodynamics simulations for the configuration considered here, we find that the free-surface does not affect the upstream turbine but significantly affects the downstream turbine.