The heavy ion synchrotron SIS18 at the GSI facility will be upgraded by a dual harmonic RF acceleration system in the process of using SIS18 as booster for the future FAIR SIS100 accelerator. The dual harmonic mode will extend the SIS18 operating towards higher beam currents. As a part of a large LLRF (Low Level Radio Frequency) upgrade of the synchrotron RF systems at GSI, new FPGA and DSP based electronics have been designed, built and commissioned. To prove the functionality of the LLRF equipment as well as the general dual harmonic topology, first machine development experiments using the existing SIS18 cavities have been performed at injection energy. In further experiments the proof of principle was shown during the whole acceleration cycle. Based on all these experiments, the main parameters of the control loops were determined. Additionally, the impact of RF gap voltage amplitude and phase variations on the ion beam have been investigated. The experiments showed a high sensitivity of the ion beam to small deviations in phase and amplitude between both harmonics and thereby confirmed the requirements on the high precision regarding the accuracy of the electronic setup especially for the closed loop control systems.