Ok, taking the best bits from a number of the above suggestions, and a couple of my own tweaks, I've come up with this:
Orc and Cass wake half-drowned on a beach in an unfamiliar world, where spirit animals grant visions, and a goddess sleeps beneath the sea, waiting for her Sun King to awaken her. With their memories shrouded by unknown magic, the teenagers must trust on instinct, and in a world of occultists and imperialist war, they are valuable sources of power to the unscrupulous.
Blackmailed by a powerful magician, betrayed by friends and hunted by a monk and his fanatic warrior apprentice, they find themselves part of a plot that will threaten the whole world.
Searching for the truth, they join an expedition to acquire an artefact from a sunken ziggurat -- an artefact, they are promised, that contains the answer to their identities.
But a power from the dawn of human thought is gathering beneath the fragile surface of the workaday world — and if Orc and Cass don’t figure out who to trust, they will be its first victims.