Historian Robert Dallek, fresh from his bestselling coup with John F. Kennedy: An Unfinished Life, which inspired headlines with its revelations about the president's health problems, is doing a dual biography of Richard Nixon and his Secretary of State, Henry Kissinger, as his next project. Tim Duggan at HarperCollins bought world rights from agent John Wright in a book that will lean heavily on voluminous papers and tapes on Nixon just emerging from the National Archives and a similar bounty of Kissinger material that the Archives obtained from the Library of Congress. Dallek, who now teaches at Boston University after many years at UCLA, will do very extensive research and has set publication for five years from now.