Drum Warning (Drums of Chaos/Jo Clayton, Bk 1)

The two worlds of Glandair and Iomard move magically together once every generation. And though the worlds never touch, people with special talents can walk between the worlds - with good intentions or with evil designs.
Cymel, a child on the world of Glandair, first realizes that danger threatens when she looks up from household tasks to see a strange wizard hovering in the air in her father's study, beckoning to her to come to him.
Lyanz, a young man on Glandair, dreams of magic that would take him away from his dreary life. A prisoner to his father's ambitions, he fears a life with no adventures and no choices - until he is magically rescued from a shipwreck by Cymel and her Iomardi aunt. Secure in Cymel's home, Lyanz finds he has much to learn about the encroaching world of Iomard and his special role in the upcoming Confluence: For Lyanz, unbeknownst to himself, is the designated Hero who can save Glandair, and thus he is the target of every evil sorcerer on both worlds.