Mamie Gummer, daughter of Meryl Streep, and Ben Walker have decided to end their marriage after less than two years, People reports.The 29-year-old Gummer (Emily Owens, M.D., Side Effects) and the 30-year-old Walker (Abraham Lincoln: Vampire Hunter) both starred in the 2008 Broadway production of Les Liaisons Dangereuses, and wed in July 2011.