The two things that I notice about the construction is (1) the frontage of the ἀποστασίαν - the accusative of what is taught (presumably to separate it as much as possible from the other accusative), and (2) that the subordinating structures commonly used with verbs of speaking - ὅτι and infinitive - are used elsewhere in the immediate context.
Does anybody have any thoughts or explanation for this syntax? Is the complexity possible in speech because of the control of the voice that is possible with the viva voce that is not possible with writing? Was this complexity the last resort? Is it a mark of high register or the best style?
More simply, and more how I would expect διδάσκειν to be used with two accusatives at a conversational level, John does use two accusatives together, but one of them is one of the weakest (least taxing to understand, i.e. either the 1st or 2nd person) pronouns for the object of the person(s) (not a great long and involved τοὺς κατὰ τὰ ἔθνη πάντας Ἰουδαίους) together with a very general (non specific - not requiring a lot of thought) object of the content of what is taught (not a very specific ἀποστασίαν ... ἀπὸ Μωϋσέως).Acts 21:21 wrote:κατηχήθησαν δὲ περὶ σοῦ, ὅτι ἀποστασίαν διδάσκεις ἀπὸ Μωϋσέως τοὺς κατὰ τὰ ἔθνη πάντας Ἰουδαίους, λέγων μὴ περιτέμνειν αὐτοὺς τὰ τέκνα, μηδὲ τοῖς ἔθεσιν περιπατεῖν.
John 14:26 wrote:Ὁ δὲ παράκλητος, τὸ πνεῦμα τὸ ἅγιον, ὃ πέμψει ὁ πατὴρ ἐν τῷ ὀνόματί μου, ἐκεῖνος ὑμᾶς διδάξει πάντα, καὶ ὑπομνήσει ὑμᾶς πάντα ἃ εἶπον ὑμῖν.