Here's #34! It is the only tie that I am allowing on the list, as it is just too funny not to have it be a tie! Click here for the master list!
The Brave and the Bold #34/Showcase #34
In 1961, the thirty-fourth issues of The Brave and the Bold and Showcase BOTH debuted Silver Age revamps of Golden Age heroes, specifically the new Hawkman and the new Atom.
Both Katar Hol (space policeman) and Ray Palmer (shrinking scientist) resulted in becoming a good deal more popular than their Golden Age predecessors, particularly Ray Palmer. Although, neither character became all THAT popular, in the sense that their individual titles did not sell all THAT well.
And in fact, ultimately, their solo titles had to be COMBINED with each other, resulting in...
Come on, when two heroes who shared a title both debuted in the same number of two different titles? That's gotta be cause for a tie!!
If I had to choose, though, I'd go with the Atom - that Gil Kane cover is an utter classic.