NaHSO4 produces H+ ion in its aqueous solution, which combines with CO23− ions (formed on ionisation of Na2CO3) to form H2CO3,an unstable weak acid.This acid readily decomposes to form CO2 gas.
2NaHSO4→2Na++2H++2SO2-4
Na2CO3→2Na++CO2-3
CO2-3+2H+→H2CO3
H2CO3→H2O+CO2
Equation of the net reaction
Na2CO3+2NaHSO4→2Na2SO4+H2O+CO2