Here's a reference to the strength of Tilden's second serve:
"The champion’s second ball was a big thorn in Johnston’s side, breaking too high and wide for him to do more than to get the tip of his racquet on it and send back a short return, which Tilden put away for a point."
(That's the NY Times report on the 1924 final at Forest Hills which Tilden won over Johnston 6-1, 9-7, 6-2).
However because of the strength of Tilden's first serve I'd be surprised if he ever had greater success on second serve.
It's tough to say because I haven't found any boxscores from back then which calculated that stat -- or even first serves successfully made.
For example the Times boxscore for this match has the game scores. From that you can calculate the points won on service:
Tilden won 60 of 86 points on serve (70%)
Johnston won 47 of 90 points on serve (52%)
... but nothing about the difference between first and second serves.