The set of U.S. Presidents that have descendants who are married to each other are Dwight Eisenhower and Richard Nixon.
Richard Nixon and Dwight Eisenhower have common descent, since the only grandchild of President Eisenhower, David Eisenhower, married in 1968 with Julie Nixon, daughter of President Nixon. The couple had three children, Jennie Elizabeth, Alexander Richard and Melanie Catherine Eisenhower, who are therefore grandchildren of President Nixon and great grandchildren of President Eisenhower.