The earliest note on the matter that I'm aware of comes from a hasty time-scheme made just as he was resuming work on the chapters 'Minas Tirith' and 'The Muster of Rohan' in 1946. One very difficult note, as part of a series of projections for the end of the story, reads
Quote:
"Aragorn ?discovers [?in lost] Palantir a ..?.. sprig of the Tree
He becomes King of M. Tirith (?wed to daughter of Elrond)"
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It's worth pointing out that Elladan and Elrohir had not yet been invented at this time.