Not really. They’re in different counties, and when I left, at least, neither had any overlap on the buses. It was a long, circuitous trip to get to Seattle from Tacoma (or vice-versa) because of that. (Unless you wanted to grab one of the express buses—but they were specially set up for travel.)
And there’s a number of townships/communities in between the cities, not least of which is Federal Way and Seatac.
