Have you ever heard the term Tacoma-Yakima used this way? I have not. In googling it I find no use of it to describe a neighborhood.
Let's go back to the authors words: Mexican gangs are causing havoc in the Tacoma-Yakima area of Washington with methamphetamine being the drug of choice for the gang. They are moving into Native American territories where a police presence is scant.
If the author were describing a neighborhood the phrase "section of the city" "neigborhood" would have been used, not "area of Washington." I think she's seriously confused.
I'm going to write her and seek clarification.