Both Johnsons were Vice Presidents and both took over when a President was assassinated.
#17 Andrew Johnson took over when Lincoln was assassinated and served the rest of the term, but was NOT reelected for a second term. So he served only a partial term. #36 Lyndon Johnson took over when Kennedy was assassinated, finished Kennedy's term and then was reelected for a second term. So he served more than 1 term.