Is common ion effect applicable on two strong electrolytes Consider the nitration of benzene using mixed conc. $\ce{H2SO4}$
and $\ce{HNO3}$.If a large amount of $\ce{KHSO4}$ is added to the
mixture the rate of nitration will be :

Faster
Slower
Unchanged
doubled


The answer given in my textbook is option 2 , i.e. the reaction will become slow. my teacher told me that it is due to common ion effect.
But in common ion effect, a strong electrolyte decrease the dissociation of weak electrolyte. So,how can common ion effect possibly be the reason here($\ce{KHSO4}$ and $\ce{H2SO4}$ both are strong electrolytes). In my view the answer should be no effect and I'd love to have my concepts about Ionic Equilibrium be corrected if I'm wrong.
Certainly not.

Suppression of dissociation of a strong electrolyte is not a very rational thing to discuss, owing to the unmeasurably large value of $K$. 

To see what I mean, try looking up the $K_{a1}$ of $\ce{H2SO4}$. 
(At least I couldn't find it!)

Your textbook's given answer is correct.

The equilibrium suppressed by the introduction of $\ce{KHSO4}$ is not the dissociation of $\ce{H2SO4}$, but rather this one:

$$
\ce{HNO3} \ + \  \ce{H2SO4} \rightleftharpoons \ce{NO2+} \ + \ \ce{HSO4-} \ + \ \ce{H2O}
$$

Nothing too auspicious: all that remains is the run-of-the-mill common ion effect we've known from Kindergarten days!

PS.
Conclusion: $\ce{KHSO4}$ sucessfully decreases the concentration of $\ce{NO2+}$, which happens to be the electrophile and a key participant in the rds(rate determining step) of Nitration of benzene.

$$
Rate=k[\ce{NO2+}][\ce{C6H6}]
$$