`Na_2CrO_4` is known as sodium chromate and `K_2CrO_4` is known as potassium chromate. If the two are mixed together, they do not react with each.
The cation in sodium chromate is `Na^+` and the cation in potassium chromate is `K^+` . The anion in both the compounds is the same, i.e. `(CrO_4)^(2-)`
For two ionic chemicals to react there should be a difference in the electrostatic forces between the ions that each of the compounds is made of. Here, that is not the case. In addition there is no compound that consists of sodium, potassium and chromate ions.