[cma]

I haven't offered any, and it would most likely be illegal if it was explicitly going on. And there is likely a wall between the different trading desks (though often the physical embodiment portion of this is literally a cubicle wall the employees can hear each other over). But the orderflow compensation doesn't need to cross over into the retail desk if the ETF itself has enough trading volume to mask some kickback without appearing too egregious.

But price fixing is also illegal--nevertheless, two gas stations across the street at a profitable intersection can engage in it solely through price signal tit-for-tat[1]. This effectively masks intentionality.

Much more fantastical and speculative: machine learning algorithms at both firms could now, or in the future, arrive at this cooperative strategy, even with the only communication being through price signals. Without any human ever even knowingly giving the explicit go-ahead.

[1] https://en.wikipedia.org/wiki/Prisoner's_dilemma#The_iterate...