Net conductivity of mixtures
This concept of electrolytic solution conductivity is complex and dependent on a variety of factors aside from mere concentration of electrolytes. There are separate laws used to describe the relationship between solution conductance and concentration dependent on the dissociative strength of electrolytes and the extent of overall solution saturation.

In your simple case, just recognize that the net conductance of the final solution may not work out to be the mere sum of conductances in the original solutions

From Wikipedia:


  The specific conductance of a solution containing one electrolyte
  depends on the concentration of the electrolyte. Therefore, it is
  convenient to divide the specific conductance by concentration. This
  quotient, termed molar conductivity, is denoted by Λm
  $$\Lambda _{m}={\frac  {\kappa }{c}}$$


Hence, since conductivity is proportional to concentration, and concentrations aren't additive, you cannot expect conductivities to be additive.