Dover's notes:ὅταν γὰρ εἰς τὸ αὐτὸ ἔλθωσιν ἐραστής τε καὶ παιδικά, νόμον ἔχων ἑκάτερος, ὁ μὲν χαρισαμένοις παιδικοῖς ὑπηρετῶν ὁτιοῦν δικαίως ἂν ὑπηρετεῖν, ὁ δὲ τῷ ποιοῦντι αὐτὸν σοφόν τε καὶ ἀγαθὸν δικαίως αὖ ὁτιοῦν ἂν ὑπουργῶν, καὶ ὁ μὲν δυνάμενος εἰς φρόνησιν καὶ τὴν ἄλλην ἀρετὴν συμβάλλεσθαι, ὁ δὲ δεόμενος εἰς παίδευσιν καὶ τὴν ἄλλην σοφίαν κτᾶσθαι, τότε δή, τούτων συνιόντων εἰς ταὐτὸν τῶν νόμων, μοναχοῦ ἐνταῦθα συμπίπτει τὸ καλὸν εἶναι παιδικὰ ἐραστῇ χαρίσασθαι, ἄλλοθι δὲ οὐδαμοῦ.
d3 εἰς τὸ αὐτὸ ἔλθωσιν 'meet', 'come together'. δ4 νόμον ἔχων ἑκάτερος 'each having a principle'. ὁ μέν...7 ὑπουργῶν: with ὁ μέν... we must understand 'thinking' νόμος involved in each case. In d7 older manuscripts have ὑπουργῶν, later manuscripts ὑπουργεῖν; if we adopt the participle, we must understand the infinitive. φρόνησιν 'intelligence'.
e1 συμβάλλεσθαι 'contribute'; cf. 185c3. εἰς παίδευσιν...2 κτᾶσθαι: if this is what Plato wrote, we must understand φρόνησιν καὶ ἀρετήν as object of κτᾶσθαι; but if εἰς were deleted (as by Schütz) παιδευσιν...σοφίαν would be the object, and the sentence would be easier to follow. e3 συμπίπτει 'comes about'.
My best guess:
For whenever the lover and the boy come together, each having a principle, the one to justly serve however he might, doing service to the willing boy, the other also to justly serve however he might, assisting the one making him both wise and good, and the one who is able contributes to the intelligence and virtue of the other, while the one who is lacking gains these through education and the wisdom of the other, then, truly, with these laws brought into one, only here in this place it comes about that the good is for a boy to accept a lover, elsewhere not at all.