Two model theory questions regarding infinitely axiomatizable classes of structures

I think it's helpful to think about problems like this from a topological perspective.

Ignoring set/class issues, consider the topology on the class $\mathbb{S}_L$ of all $L$-structures generated by the base $$\{\{\mathcal{M}: \mathcal{M}\models\varphi\}:\varphi\in Sent_L\}.$$ Trivially we have "axiomatizable $\iff$ closed." Much more significantly, by the compactness theorem we have "finitely axiomatizable $\iff$ clopen," and in fact this is just a restatement of the compactness theorem. (Note that if we were to do this with a non-compact logic in place of $\mathsf{FOL}$ all we would be guaranteed is "finitely axiomatizable $\color{red}{\implies}$ clopen.")

The fact that we can topologically characterize finite axiomatizability immediately gives a strong negative answer to your second question on general topological grounds: if I partition a clopen set into two open pieces, each of those pieces is clopen. However, the first question is left untouched by this reasoning and in fact ultimately has a positive answer:

Consider the language $L=\{A,B\}$ consisting of two unary relations. Let $K$ be the class of $L$-structures in which there are at least as many $A$s as $B$s, and let $K'$ be the class of $L$-structures in which there are at least as many $B$s as $A$s. We have $$K\cup K'=\mathbb{S}_L,$$ but it's easy to show that both $K$ and $K'$ are infinitely axiomatizable.