David Muir will start to lead breaking-news duties at ABC News, a new effort by the Walt Disney unit to rework an unorthodox arrangement involving two of its top anchors. Under a new structure, Muir will largely handle the bulk of ABC News’ special reports and news of national importance, while George Stephanopoulos, the “Good Morning America” and “This Week” anchor may shepherd similar efforts in the morning, according to three people familiar with the matter. Since 2014, when Diane Sawyer stepped down from her tenure on “World News Tonight,” Stephanopoulos was recognized as ABC News’ “Chief Anchor,” and had led coverage of events of critical importance, presidential debates and elections. He has often been paired with Muir and Linsey Davis, an anchor who leads a program on the network’s streaming-video hub, ABC News Live. That title will no longer be utilized.