Luke-Acts Knowledge of Matthew

Stephen Carlson points to a very interesting parallel between Matthew 3.11 and Acts 19.4, both of which juxtapose the themes of repentance and the "coming one" in John the Baptist's preaching. This is in contrast with Mark, Luke's Gospel and John. Given that, on the standard Two-Source Theory, Luke-Acts is independent of Matthew, this is striking. So how could it be explained? Stephen lays out the evidence and then asks:

Of the four gospels, only Matt 3:11 juxtaposes, as does Acts 19:4, the motifs of a baptism of repentance (John) with the one coming after (Jesus). According to the Critical Edition of Q (p. 14, at Q 3:16b, which aggressively adds Jesus's baptism to Q because of too many minor agreements), the phrase "for repentance" is Matthew's redaction of Q's baptism. In other words, the author of Luke at Acts 19:4 knows Matthew, or, if the editors of CEQ are wrong, Q is more like Matthew than we thought.

As a defender myself of the theory that Luke (the author) did know Matthew, what I like to do on such occasions is to try to get into the Q theorist's shoes. How would I answer this if I were persuaded of Q? (I always try to test my own arguments by trying to find the best possible arguments against them. This is not because of some kind of schizophrenia but because it can help one to sharpen up one's arguments or, sometimes, to drop them before it's too late). What I think I
would say here would be that there is a third option:

(1) It is not that Luke knows Matthew -- we know that that is not possible for a variety of reasons, chief among which are (a) Luke's eccentric editorializing that would be implied by that theory & (b) the phenomenon of alternating primitivity in double tradition. (My hypothetical Q theorist has not, unfortunately, read The Case Against Q or, if s/he has, s/he is -- God forbid! -- unpersuaded by it).

(2) And it cannot be that Q is more like Matthew than we previously thought. If Q had featured repentance here, Luke would have carried it over so producing the same juxtaposition of repentance + coming one in Luke 3. After all, we know that Luke has no aversion to repentance -- it is a favourite in his Gospel (e.g. Luke 5.32R, 15.7 QD, 24.47). On the other hand, repentance is something Matthew might have added in Matt. 3 (e.g. cf. the prominence the theme is given in Matt. 3.2). So Q did not have repentance here.

(3) So where did Luke get it from in Acts 19? It was probably his memory of these two features, both of them congenial, from Mark and Q. John's baptism of repentance is a key feature of Mark 1; the announcement of the coming one is a key feature of Q 3 and Q 7. So he juxtaposes them himself in Acts 19 in the same way that Matthew juxtaposed them himself in Matt. 3. So there is nothing here that cannot be explained by independent redaction.