Is the union of two elementary classes an elementary class?

Question

Suppose $K$ and $K'$ are two classes of structures in the same language which are both elementary classes. Is their union also an elementary class? Note, the classes do not have to be finitely axiomatizable, they just have to be axiomatizable.

Answer 1

If $K$ and $K'$ are axiomatized by the sets $\Sigma$ and $\Sigma'$ of sentences, then $K\cup K'$ is axiomatized by $\{\alpha\lor\beta:\alpha\in\Sigma\text{ and }\beta\in\Sigma'\}$.