The urban legend debunking site Snopes has this to say about one of the better known Lincoln-Kennedy similarities:
Lincoln's secretary, Kennedy, warned him not to go to Ford's Theater. Kennedy's secretary, Lincoln, warned him not to go to Dallas.So...no Lincoln secretary named Kennedy, eh? Well...case closed, then.
This is one of those coincidences that isn't a coincidence at all---it's simply wrong. John Kennedy did have a secretary named Evelyn Lincoln (who may or may not have warned him about going to Dallas), but one searches in vain to find a Lincoln secretary named Kennedy. (Lincoln's White House secretaries were John G. Nicolay and John Hay.)
But...what if another possibility exists?
In the December 2008 issue of Civil War Times Illustrated, an article by Lincoln scholar Harold Holzer, extracted from a full length book about Lincoln's time as president-elect (November 1860-March 1861), addressed Lincoln's incognito passage through pro-secessionist Baltimore, Maryland on the way to his inauguration. Concerned about threats against the life of the president-elect, Lincoln's aides and friends convinced him to "sneak through" Baltimore by night rather than make his scheduled daytime appearance there.
This passage, describing one of three letters of warning that Lincoln received while stopping over in Philadelphia, caught my eye:
So here we do, in fact, have a Kennedy who, at one point in what was in all probability a life of obscurity, warned Lincoln about a threat to his life, although obviously not the same threat familiar to history. In the end, Lincoln made the journey safely, was sworn into office during the first week of March, 1861, and led the country through four years of civil war before finally falling victim to John Wilkes Booth's bullet in April 1865.
"Stone's report completed the package and quoted a New York 'detective officer' who had been on duty in Baltimore for three weeks and now believed 'there is serious danger of violence to and the assassination of Mr. Lincoln in his passage through that city should the time of that passage be
known.'...Unmentioned---it would have meant little at the time, but a great deal just a few months later---was the fact that the 'detective officer' in question, John A. Kennedy, superintendent of the New York Metropolitan Police, had taken his discoveries to Stone...Stone had rushed Kennedy's warning to Scott, an alarmed Scott had shared it with Seward, and Seward had summoned his son and entrusted him to speed it to Lincoln 'wherever he is.' (emphases mine)"
Is it possible that somewhere along the way, someone (an obscure Lincoln scholar perhaps?) might have mentioned Detective Kennedy's name in context with this earlier assassination attempt on Lincoln, perhaps to an equally obscure JFK scholar, who could then have referenced the name of Kennedy's secretary Evelyn Lincoln? Might this "truth" about the crossed paths of two presidential assassinations have arisen from a careless mangling of what were, on the surface, indisputable facts?
The possibility tantalizes, does it not?
Still, we can never know for certain.