I am trying to translate the following passage from a Baptismal Oration of St. Gregory the Theologian.
τριῶν ἀπείρων ἄπειρον συμφυΐαν, Θεὸν ἕκαστον καθ' ἑαυτὸ θεωρούμενον, ὡς Πατέρα καὶ Υἱὸν, ὡς Υἱὸν καὶ τὸ ἅγιον Πνεῦμα, φυλασσομένης ἑκάστῳ τῆς ἰδιότητος. Θεὸν τὰ τρία σὺν ἀλλήλοις νοούμενα, ἐκεῖνο διὰ τὴν ὁμοουσιότητα, τοῦτο διὰ τὴν μοναρ χίαν.
My question is, to which statement does the ἐκεῖνο and τοῦτο refer here? From looking at a paraphrase, it appears the ἐκεῖνο refers to 'Θεὸν ἕκαστον καθ' ἑαυτὸ θεωρούμενον,' and τοῦτο refers to 'Θεὸν τὰ τρία σὺν ἀλλήλοις νοούμενα.' Is this correct?
Thanks!

You can locate both of your references inside the same phrase: ἐκεῖνο [Θεὸν] διὰ τὴν ὁμοουσιότητα, τοῦτο [σὺν ἀλλήλοις νοούμενα] διὰ τὴν μοναρχίαν. The meaning is that each of them is the God because of their common divine essence, and that they are perceived together (σὺν ἀλλήλοις τὰ τρία) because they are only one God (μοναρχία).

As Browne and Swallow translate, "the infinite conjunction of Three Infinite Ones, Each God when considered in Himself; as the Father so the Son, as the Son so the Holy Ghost; the Three One God when contemplated together; Each God because Consubstantial; One God because of the Monarchia. No sooner do I conceive of the One than I am illumined by the Splendour of the Three; no sooner do I distinguish Them than I am carried back to the One." (See www.elpenor.org/gregory-nazianzen/holy-baptism.asp?pg=27" target="_top">Gregory the Theologian, On Holy Baptism - Oration XL, p. 27)

So the ἐκεῖνο does refer back to the Θεὸν ἕκαστον καθ' ἑαυτὸ θεωρούμενον?

You have Θεὸν in the same phrase: Θεὸν τὰ τρία σὺν ἀλλήλοις νοούμενα, ἐκεῖνο διὰ τὴν ὁμοουσιότητα, τοῦτο διὰ τὴν μοναρχίαν.