Manhunters of Mars and a Pair of Johns

J'onn J'onzz and Roh Kar were both products of the 1950's. In 1953, Roh Kar teamed up with Batman and Robin of Earth-Two (in Batman #78). J'Onn J'onzz first appeared in an adventure   on Earth-One (in Detective Comics #225). According to the Batman Encyclopedia: "Roh Kar arrived on Earth-Two and was aided by Batman in apprehending the renegade Sax Gola". 

Earth-Two's Batman had previously visited Mars but never had occasion to meet this Manhunter from Mars. On Earth-One, J'Onn was accidentally pulled from his native world by a teleportation device created by Professor Erdel. Possessing superhuman powers on Earth-One, J'Onn posed as the Martian Manhunter.

While J'onn had numerous numerous adventures on Earth-One, both in his adopted home of Middleton and alongside his teammates in the Justice League, Roh only had one chronicled adventure on Earth-Two with Batman and Robin. Although his physical descriptions matches that of the mystery Martian who would years earlier welcome the Dynamic Duo's Justice Society colleague, Al "Atom" Pratt, to Mars. On that world, the Atom gained superhuman powers comparable to those displayed by Jon'zz. One attributed in common was the ability to form mental bonds with others. J'onn manifested this naturally, as had all Martians in his universe. Roh relied on technology such as the Educatograph, which he used on Pratt to teach the young man the Martian language while visiting. That bond seemingly endured for quite sometime following the Atom's return to Earth-Two, when he began developing superstrength due to exposure to radiation.

Roh had to rely on a jet pack and technology to give him the edge in battles with the criminal element. Both of these became known as the Manhunters of Mars One and Two. Like J'Onn and his brother T'Omm, Roh fought crime on Earth. Concurrent to this, J'onzz's civilian alias on John Jones operated as a policemen with hidden powers similar to Larry Trent, a man who's alter ego was for a time adopted by the Superman of Earth-Two which inspired Trent towards crimefighting as a patrolman of Radio Patrol.