Up to 6 Schottel Instream Turbines (SIT250) can be mounted on the tidal platform PLAT-I developed by Sustainable Marine Energy. Due to the close proximity of the turbines interactions can occur between them. Two horizontal axis tidal turbines in model scale are investigated experimentally and numerically to analyze these interactions. Experimental data were measured in a towing tank and consist of integral values for torque, thrust and rotational speed. Both a steady state and an unsteady three dimensional Reynolds Averaged Navier Stokes (RANS) approach are utilized for simulating the turbine flow field. The first part of the paper compares simulation results of a single turbine at different tip speed ratios with measurements to validate the numerical approach and its employed models. The second part analyses the interaction between two turbines. The axial distance in main flow direction between the turbines is half the rotor diameter. The radial distance measured between the hubs of the turbines is varied in steps of 0.2 between 0.0 and 2.0 times the rotor diameter in the experiment and between 0.0 and 1.4 in the simulations. Measurements were conducted for tip speed ratios of 3, 4 and 5. In the simulations the tip speed ratio was fixed at 4. The used simulation domain replicates the actual width and height of the towing tank and a sufficient length up- and downstream of the turbines. The water surface is modeled with a free slip wall. Both thrust and torque are compared between simulation results and experimental data. Furthermore, a detailed analysis of the results and flow field in the numerical simulations is presented and the interaction between the turbines is discussed.