Usually, when the Fastest Men of Earths One and Two met, it wasn't purely a social visit. Nor did these meetings generally start out as such... and certainly this adventure was a prime example of this. Barry Allen woke up, only in another bed on another world, having assumed Jay Garrick's place. Jay-Flash had himself disappeared, leaving Barry-Flash to impersonate him and accept an award from the grateful citizens of Keystone City. After this, Barry encountered a mysterious man known as Dorvan who revealed that he knew where Jay currently was... in a temporal limbo. Providing Barry the means to enter that timestream to save Jay, Dorvan afterwards removed his disguise to reveal that he was the former foe of each Flash, Vandal Savage. Only now, Savage was aging tremendously since their last encounter, since his the effects of his immortality were gradually removed. A possible reason for this is due to another comet that slammed into the sun of Earth-Two, which irradiated the atmosphere causing Jay-Flash to use gas from meteor-fragments to absorb this toxix radiation. And, no doubt, also removed Savage's ageless ability as well!
Meanwhile, if such a concept exists in the timestream, Barry-Flash and Jay-Flash travel through an exotic obstacle course until they meet its protector Tempest. After besting the temporal protector, Tempest sends the Scarlet Speedster on their way to find Vandal's comet... the same comet that struck him centuries previous and bequeathed him with his immortality. Apparently, Vandal had implanted in Jay-Flash's mind the desire to bring the comet back to Earth-Two, only another speedster's energy was also needed to accomplish this. And so, the dynamic duo successfully steered the object back into Jay-Flash's time/space, and followed it back to Earth-Two. They were to late to stop it from striking Savage, although the effects of the comet caused their ageless adversary to disappear.
Never fear, for Vandal Savage would return, stating that the effects of the comet had not transformed him into a phantom on his native world and solid on Earth-One. Hence, he hatched an elaborate scheme to lure Barry-Flash and that world's Green Lantern, Hal Jordan, into a confrontation with him by using Iris Allen And Carol Ferris as bait. Venturing to Earth-Two to consult with Jay-Flash, that speedster informed his friends of Savage's perdicament but was unable to assist them further. Seeking to find their women, the two Justice Leaguers tracked down an artificial fireball like that which granted Vandal near endless life ages prior, with one of the girls inside. Thanks to their powers and tenacity, the pair were able to free Carol Ferris although Savage's plan fell apart and he was captured.
Meanwhile, the reason why Jay-Flash had to remain on Earth-Two was to watch over a woman... Iris Allen... who needed to remain there for her own wellbeing? Why? That is a tale for another day.