Since Spartan Kings behave as if they are promoted Hoplites, and thus demote on capture, this situation would require promoting a passed Hoplite. The rule description does not specify whether this is allowed. In Spartan Chess there is a restriction on when you can promote to King, and it is not obvious how this rule should be extended to the case where Hoplites fight in an army headed by a Persian King. One possible extention would be that a Persian King already counts as a pair in itself, and that its presence thus suppresses the King option on promotion. Then the situation could never occur.
Since Spartan Kings behave as if they are promoted Hoplites, and thus demote on capture, this situation would require promoting a passed Hoplite. The rule description does not specify whether this is allowed. In Spartan Chess there is a restriction on when you can promote to King, and it is not obvious how this rule should be extended to the case where Hoplites fight in an army headed by a Persian King. One possible extention would be that a Persian King already counts as a pair in itself, and that its presence thus suppresses the King option on promotion. Then the situation could never occur.