The focus of this experimental study is to investigate the effects of winglets on the performance of two interacting similar horizontal axis model wind turbines. For this purpose, a downwind winglet is designed and manufactured to be attached to the blade tips of the upstream turbine. A set of wing extensions with the same length as the winglets is also produced to be compared to the winglets. Power and thrust coefficients of both turbines are measured with winglets as well as with wing extensions attached to the blade tips of the upstream turbine and are compared to the baseline case (rectangular tip without any tip devices). The model turbines are three bladed and have a rotor diameter of 0.94 m. The measurements are performed in two different wind tunnels (closed test section and open jet). For both sets of measurements, winglets have a noticeable increasing effect on the power coefficient of the individual turbine. There is an increase in the thrust coefficient as well. Measurements on the second turbine are done while it is positioned at downstream locations in line with the upstream turbine. Results show that it produces less power while operating in the wake of the upstream turbine with winglets. However, the overall power efficiency of two turbines can increase for the wingletted case. Moreover, results with wing extensions show that although upstream turbine produces more power with wing extensions attached, the power coefficient remains the same as the baseline case due to growth in rotor swept area and hence, it is less than the power coefficient of wingletted turbine.