If you wanted to just strengthen the current/theatrical plan, you could have Palpatine say something to Rey like, “Only the strongest can rule.”

Basically, having them fight only benefits Palpatine. If one kills the other, then nothing would stand in the way of him possessing the survivor. And whoever survives would be the stronger vessel. And Palpatine frames it as a test. Testing their skill and strength to see who can be the rightful heir.

“I never wanted you dead. I only wanted the strongest to rule.”
“Only the strongest can take the throne.”
Idk, something like that.