Interpublic is merging two of its agency brands, Lowe & Partners and Mullen Advertising, into the Mullen Lowe Group. Alex Leikikh, formerly the chief executive of Mullen, will lead the combined entity as worldwide chief executive. Michael Wall, previously chief executive of Lowe & Partners, will exit the company after a brief period. The merger was spurred by Mullen’s need for a more expansive network to serve clients globally, Leikikh said.