Why is this temple dedicated to both deities? I can't find a connection between them in either myth or theme.
Well, it was a temple that just had two separate reasons to be built.
Since Poseidon was involved in Athens-Elusis relationship, and Artemis was the Propylaea(1) of Eleusis, so they made one temple to honor both of them.
(1) prop·y·lae·um- the entrance to the Acropolis at Athens.