Interesting theory...
Like most theories, I suppose we can't prove or disprove it unless the never-quite-exhausted notes of Tolkien someday yield something to the effect of "awesome statue--but two would be better! Must include in the Hobbit sequel..."
Given Tolkien's own comments on the Númenóreans, namely that they were rather like the ancient Egyptians in their taste for megalithic sculpture (and over death, but that's less relevant here), I've always thought of the temple of Rameses II at Abu Simbel as something of an inspiration: matching megalithic statues of royal power on the banks of the river, but given a greater dramatic punch by being put on either bank, almost Colossus of Rhodes style, so that those entering the realm of Gondor would have to pass between.
But, of course, Tolkien himself notes the difficulty or impossibility of finding the sources for ideas, some of which have decomposed in the mind completely.