"Flash of Two Worlds" was the story that re-introduces the Golden Age into DC continuity through the concept of "Earth-Two".  Originally published in DC's The Flash No.123  1963 (see scan below) and in Australia in The Hundred Comic No.65.  The Silver Age Flash, Barry Allen travels to Keystone city - home of his comic book idol Jay Garrick the Golden Age Flash. This adventure brings Jay out of retirement.

In Australia Hundred Comic Monthly, with Five- Score Comics Monthly & Century Comics monthly  introduced the concept of 100 (and with All Favourites & Mighty Comics later 116) page comics. Publisher Colour Comics alternated The Flash & Wonder Woman as leading character. Started in 1959 this doubled the price of comics to 2/- (two shillings) but tripled the pages!!  A concept used in North America by DC over a decade later.

"Double Danger on Earth"  - Hundred Comics No.75 (originally Flash 129) introduced Jay Garrick to Earth One and included a flashback of All Star Comics No.57 (the last GA JSA adventure).

A few months later the JSA were revived in Flash 137 (Hundred Comic No.86) and then came the classic Crisis on Earth 1 and Crisis on Earth 2 were published in Justice League of America 21 & 22.