James Adam, The Republic of Plato

ἂν μὲν τοίνυν κτλ. The alternatives are between continuous speech and dialectic. By λόγον in παρὰ λόγον Thrasymachus' speech in 343 A ff. is meant: to this Socrates would reply, after which Thrasymachus would speak again, and finally Socrates. Thus each party would have delivered two speeches. In Athenian lawsuits there were often two speeches delivered by the accuser and two by the defendant (Meier und Schömann Attische Process p. 924), so that Plato's imagery is borrowed from the law-court, whence δικαστῶν τινῶν τῶν διακρινούντων just below. This point escaped Ast, who reads καὶ αὖθις οὗτος ἄλλον ἡμῖν (after Ficinus and Stephanus).

ἀντικατατείναντες is intransitive: cf. II 358 D κατατείνας ἐρῶ τὸν ἄδικον βίον ἐπαινῶν and 367 B: the notion (as in ξυντείνω, ξυντεταμένως and the like) is of nervous tension. The word cannot mean ‘replying to one another in set speeches’ (J. and C.). “Setting out alternative lists of advantages” (remarks Bosanquet) “was the well-known method of fable or poetry. See Book II” 361 D—362 C and 362 E— 365 A: “and compare Prodicus' Choice of Heracles (Xen. Mem. II 1) and the discussion between the Just and Unjust arguments in the Clouds of Aristophanes.”