Neville Rogers informs us that Shelley's "soul of my soul" idea was persistently prevalent in his poetry and thought throughout his literary career: "Three more early poems, written in 1810 at the age of eighteen and entitled 'To Mary,' lament the suicide by poison of a maiden whose story, an unhappy love affair, purports to be the experience of a friend of the poet--an anticipation, it might seem, of the device that was to be used with Alastor and again with Epipsychidion" (38).
Consider the following passage from Alastor which illustrates Rogers's point:
Her voice was like the voice of his own soul
Heard in the calm of thought; its music long,
Like woven sounds of streams and breezes, held
His woven sounds of streams and breezes, held
His inmost sense suspended in its web
Of many-colored woof and shifting hues. (153-58)
This passage also reminds me of the narcissistic tendencies in Epipsychidion, which Barbara A. Schapiro points to in her discussion on lines 45-52.
Rogers, Neville. Shelley at Work: A Critical Inquiry. 2nd ed. Oxford: Clarendon P, 1967.