Saber & Shadow by S.M. Stirling; Shirley Meier

From the authors of The Cage. In the Fifth Millennium, there was no city larger or more corrupt than Illizbuah. The intrigues of its priests and lords, wizards, merchants and thieves were ancient and inbred and as formal as a dance. Here two homeless wanderers would set all their plans at naught and the city--and the world--would never again be the same.