If anything, they seem like buds. I hope their story ends similar to a naruto and nine-tails situation, where they're fighting with eachother constantly at first, and then become good friends. I'd hate to see edgy mcreggie fighting evil redisthecolorofevil warlord to the death.
Well the idea of the Darkin is that the weapon attempts to overtake the user. That's how the duality of Rhaast and Kayn exists. They're both vying for control over the other, in that sense sure they're enemies, but if you think about it. If you were stick with someone you were competing against all the time you would likely end up with some friendly banter back and forth similar to how Rhaast and Kayn are.
They are like minded. Both ambitious capable fighters. Rhaast is sometimes impressed with Kayns' abilities and Kayn appreciates the power Rhaast gives him but at the end of the day one of them has to die so yeah they are still symbiotic foes.
If there's a way to restore a Darkin's body and Rhaast and Kayn didn't have to vie for control of one another, they could definitely be good friends. Sadly, that doesn't seem to be the case, unless our favorite Targonian Aspect 
knows a way to reverse the imprisonment she taught.
They really are, at the end of their journey there's only room for one at the destination
I think how that rhaast has “chose” Kayn they’re are bound together forever, it’s one or the other Rhaast consumes Kayn, or Kayn takes over Rhaasts power. They can’t stay split like that forever. I mean look at Kayns model. One of his arms is all armored and weird. Like rhaasts. They’re constantly battling each other for control
Kayn is using Rhaast as a tool, Rhaast is enemies to everyone. He has no friendship for Kayn and would gladly consume him.
So best buds it seems. :)
I mean, they're as friendly as two people trying to kill each other can be
I would love nothing more than for their story, in defiance of all the grimdark it's featured, to end with the Power of Friendship.
