For decades, antitrust lawyers at the U.S. Justice Department and the Federal Trade Commission have squabbled over turf, but now the two agencies are moving toward a rare level of harmony.

One key: William Baer and Deborah Feinstein. For the first time ever, two lawyers from the same firm stand at the top of federal antitrust enforcement, bringing to their jobs a rapport built over 27 years working on and off together at Arnold & Porter.