Barbara Matusow describes that transitional format in her book The Evening Stars: The Making of the Network News Anchor (1983):
"[Arledge's] most immediate problem was what to do the incompatible team of Barbara Walters and Harry Reasoner. Arledge's initial move was to buy time by making format changes designed to minimize their impact on the Evening News while he was working on a replacement formula. "Two-shots" were eliminated, meaning that Walters and Reasoner were not shown together on the screen, and something called a "whip-around" was introduced -- correspondents handing off their reports directly to each other instead of going back to the anchor for introductions. Subanchors began to appear: Frank Reynolds was featured prominently in Washington, as was Peter Jennings from various European capitals."
(In this case, Barbara Walters was off and Sam Donaldson served as the Washington subanchor. When WNT premiered, of course, it was permanently anchored by Reynolds in Washington, Max Robinson in Chicago, and Jennings in London. It's also interesting to note that the distinctive red corner stripe was introduced before WNT was launched.)