Titration of a polyprotic acid with a strong base

My book says:



> 
> in the titration of a polyprotic acid we have more than one equivalence point which represents the neutralization of each step of the acid ionized.
> 
> 
> 


The reaction might be:


$$\ce{H2CO3 + NaOH -> NaHCO3 + H2O}$$


$$\ce{NaHCO3 + NaOH -> Na2CO3 + H2O}$$


Now my questions are:



> 
> 1. Why does the reaction of second step in the ionization of acid
> does not start until its first step has been consumed totally?
> 2. Even if it does (may be), why we do not consider it?
> 
> 
> 


I think of a possible answer, which I have applied to and got fairly well results. I think It might be that the reaction of multiple steps proceed simultaneously but we can, in a good manner, break up all these steps into independent ones such that they start only after the previous one has completely taken, and do calculations with it, and then make it overlap such that it remains the same as it was.


I call this the "Overlapping of Independent steps to work simultaneously".
Is it a correct method to think of?


