If both members of the conjugate pair are dissolved

In a solution where both members of an acid–base conjugate pair are dissolved in comparable concentration, their reaction with water to reach equilibrium proceeds to a lesser extent than in solutions where only one of these is a major species.

This is because, in both cases one product of reaction (the other member of the conjugate pair) is a major species.

acid + H₂O conjugate base + H₃O⁺

conjugate base + H₂O acid + OH^–

As a result, [H₃O⁺] is lower in a solution where both the acid and the base are major species, than in a solution where only the acid of the pair is a major species.  

Similarly [OH^–] is lower than it would be in a solution that has only the conjugate base as a major species. 
 

The small extent of reaction of both solutes with water means that the equilibrium concentrations of both the acid and conjugate base are very well approximated by the initial concentrations.  Thus calculation of pH of such a solution from pK_a is very straightforward as the quantities highlighted are known.
 
This smaller extent of reaction to reach equilibrium also means that for a solution in which both members of a conjugate pair are major species,