There are 4 books in the Quarters series. The newest book is The Quartered Sea which came out in 1999.

The Bards of Shkoder hold the country together. They, and the elemental spirits they Sing – earth, air, fire, and water - bring the news of the sea to the mountains, news of the mountains to the plains. They give their people, from peasant to king, a song in common.…
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2
Fifth Quarter

Fifth Quarter
Planning to assassinate the evil Gyhard, siblings Bannon and Vree of the Havalkeen army face a terrible choice when Gyhard steals Bannon's body and forces the pair to coexist in Vree's body

3
No Quarter

Book cover of No Quarter
Bannon and Vree, a brother-and-sister assassin team, must share Vree's body when Bannon's is stolen from him and begin a desperate quest into a neighboring kingdom in order to regain Bannon's body from Gyhard. Original.

4
The Quartered Sea

The Quartered Sea
When an expedition into the uncharted waters to find the homeland of the legendary Dark Sailor ends in disaster, Benedikt, a young bard, finds himself stranded in a mysterious unknown world, the pawn in a deadly game of politics and religion that destroy a kingdom's precarious balance of power. Original.