The mixing performance of a dual shaft mixer with a Rushton turbine installed on each shaft was investigated experimentally and numerically in the laminar regime using viscous Newtonian liquids. A mixing quantification method based on the analysis of a fast acid-base discolouration reaction was used to follow the macro-mixing evolution in the tank. The macro-mixing curves generated by this method were used to determine the mixing times as well as to reveal the presence of flow compartmentalization and dead zones. In comparison with to a dual impeller system with two Rushton turbines on the same shaft, it is shown that the dual shaft mixing system can eliminate the flow compartmentalization, break the segregated regions and reduce the mixing time. Experimental results show that the counter-rotating mode of the impellers give smaller mixing times than the co-rotating one. A mixing time correlation was developed and used to characterize the strong interaction between the impellers when operated in counter-rotating mode. Numerical simulation results were used to highlight and explain the difference in global performance between the two rotating modes.