I just found it. It was Curse of the Mistwraith, first in the Wars of Light and Shadow series by Janny Wurst.
From the Wikipedia summary:
Born on a splinter world, Lysaer and Arithon are half-brothers raised apart in enmity. Cast through a Worldsend Gate, they arrive in Athera, the ancient world of their ancestors cloaked in the fog of the malicious Mistwraith. Found by the Fellowship of Seven and urged to fulfill a prophecy which will free Athera from the Mistwraith and allow the clans of the Old Bloodlines to rule again.
and the cover image there shows the two brothers:
