why do we use two different strains of bordetella pertussis in the production of its vaccine?

Answer 1

Using multiple strains to manufacture any vaccine ensures a wider immune response and formation of a greater range of antibodies against the pathogen.

In this case, because you can have B. pertussis and B. parapertussis infections, why not protect against both of them in one shot? It's just more efficient to combine them into one vacccine.