"What would the "two natures" be?"
You know, Lucretius uses almost that exact phrase;
Sed neque Centauri fuerunt nec tempore in ullo
esse queunt duplici natura et corpore bino
ex alienigenis membris compacta, potestas
hinc illinc partis ut sat par esse potissit.
id licet hinc quamvis hebeti cognoscere corde.
But Centaurs ne'er have been, nor can there be
Creatures of twofold stock and double frame,
Compact of members alien in kind,
Yet formed with equal function, equal force
In every bodily part- a fact thou mayst,
However dull thy wits, well learn from this:
There are interesting parallels here; in the above passage from book 5 Lucretius is concerned with the question of tracing the living thing back to its proper seed. In a separate passage in book 4, the poet uses the Centaur to make a different point;
nam certe ex vivo Centauri non fit imago,
nulla fuit quoniam talis natura animata;
verum ubi equi atque hominis casu convenit imago,
haerescit facile extemplo, quod diximus ante,
propter subtilem naturam et tenvia texta.
cetera de genere hoc eadem ratione creantur.
quae cum mobiliter summa levitate feruntur,
ut prius ostendi, facile uno commovet ictu
quae libet una animum nobis subtilis imago;
tenvis enim mens est et mire mobilis ipsa.
For soothly from no living Centaur is
That phantom gendered, since no breed of beast
Like him was ever; but, when images
Of horse and man by chance have come together,
They easily cohere, as aforesaid,
At once, through subtle nature and fabric thin.
In the same fashion others of this ilk
Created are. And when they're quickly borne
In their exceeding lightness, easily
(As earlier I showed) one subtle image,
Compounded, moves by its one blow the mind,
Itself so subtle and so strangely quick.
In the first passage, Centaurs are declared not to exist because no such disparate seeds as those which produce men and horses could conceivably commingle in the way necessary to produce a man-horse. In the second passage he is giving an explanation of how the legend came to be in the first place; it was not seeds of men and horses that mingled and produced the centaur, but images of men and horses that mingled and produced the illusion of the existence of the centaur. These, the same images which we depend upon for knowledge of the gods?
Edit; I see Bryan and I crossposted with reference to the two passages on centaurs.