Open betrayal between X and RC will not occur before the goal is literally within sight.

Xykon is savvy enough to always give Redcloak reason to believe he will work his end of the plan. Xykon has no reason to back out of a plan that he believes will give him control over a Gate.

Since Redcloak knows that the Ritual itself betrays Xykon, Redcloak does not care much about the likelihood of betrayal by Xykon after the Ritual.

They are both committed to race to the same goal together. Xykon thinks he is powerful enough to end the relationship on his terms, if he so chooses, after their quest is finished. Xykon happens to be twice wrong about the endgame.