This is false even in the case you describe as it would imply Cantor-Schroeder-Bernstein for fields. One can take the standard counter example for proof. Let $F_i = \mathbb{C}(X)$ and $K_i = \mathbb{C}$ for all $i.$ Then $K_i$ injects into $F_i$ and $F_i$ injects into $K_i,$ but $F$ and $K$ are not isomorphic, as $F$ is not algebraically closed.
JSpecter
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