Crisis (DC Comics) - "Crisis Between Earth-One and Earth-Two!"

"Crisis Between Earth-One and Earth-Two!"

The JLA and JSA must fight the Anti-Matter Man when reality warps caused by a lab assistant of Ray Palmer's sub-space machine causes people to randomly shift between Earth-One and Earth-Two, including Solomon Grundy, Blockbuster, and some of the superheroes, as well as causing the two Earths to move towards each other in Warp space meaning the Spectre has to hold them apart but is weakening due to the presence of antimatter. The machine is turned off, causing Grundy and Blockbuster to switch Earths. But Doctor Fate gets a message from the Spectre, and transports the heroes to Warp-Space. The superheroes battle Anti-Matter Man and are able to stun him. The Atom is able to find a way to get the Earths apart. He tells the Spectre if he shrinks down to an inch in height, then suddenly expands again, the Earths will be thrown apart, though the Spectre will probably be destroyed. The Spectre agrees to this and is blown apart. However he uses an electro-magnetic field to reform himself, and the shockwaves send Anti-Matter Man back to his Universe. The second story in this tale is called "The Bridge Between Earths!"

The stories can be found in Justice League of America #46 and 47 published in 1966.