If you remove Nixon's assassination, you introduce butterflies which will likely affect the Kennedy assassination. There's nothing in the universe forcing Kennedy to be shot, hospitalized and in a coma and dying of his wounds later as he was, or nor alternately killed by an exploding plane or a deranged Palestinian or whatever possible death scenario one could dream up.
Maybe without the assassination of Nixon, the Secret Service would be less focused on stopping a lone gunman with a pistol, and give more weight to a potential sniper attack? That could prevent Kennedy's assassination.