Since you measure both the power and mixing time, you already have the answer.  The power number is just the power at the speed and impeller diameter, the same as for one impeller.  If the impellers are different sizes, pick the diameter of one, but remember which one you used for the diameter.  You can use the new power number the same way you would use the power number for a single impeller.  The only difference is that the power number for two impellers will be larger than the power number for either impeller separately.  If the impellers are of equal size and type the total power number for both impellers on the shaft, will be approximately two times the power number for one impeller.
 
The same type of analysis can be done with mixing time.  If the impellers are of the same size and type, the mixing time will be two thirds to three fourths of the mixing time for a single impeller of the same type and size.  Using the results will be the same as if you had just one impeller.
 
The only real difference, which can also be measured, is a condition where only the lower impeller is submerged in the liquid.  In that case, you can measure both power and mixing time for one impeller.  By difference, you can estimate the power number of the other impeller and distinguish the difference between one impeller and two impellers.

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