Is common ion effect applicable on two strong electrolytes
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.

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