Actually the book makes this reference from the movie much better...Vito Corleone before the Maranzalla war keeps Clemenza in Queens (under his thumb, because he needed to rein him in) and gives Tessio Brooklyn. He further instructs Tessio (and not Clemenza) that the two should never be seen together. MP elaborates that Tessio knew that Vito did not want the two of them to conspire against him, and also to show enemies that Tessio was a splinter group. He gave more lattitude to Tessio, and if you recall after the shooting of Vito at the estate, Tessio's men guard the compound. In the book, when Sonny makes the call to Tessio, Tessio's reply is "you know my men?" meaning that even up till that point, Tessio is given more power than Clemenza, even though they are both Capos