Jon the Viking Prince existed on both Earths One and Two. How do we know? The Lord of Time brought back Earth-One's Jon to the present to attack the Justice League, and visiting Justice Society, in order to reverse a destructive device he created. The other Jon was brought forward into the modern era in order to combat a threat to Earth-Two's Cape Canaveral. On both occasion, the Princes worked alongside many of history's heroes.
One of these men was given the name Jon from a legendary prince of old, and worked alongside Captain Olaf and his daughter Gunnda and fought Baron Thorvald who sought to eliminate Jon as a potential heir.
The other Jon was born with that name, son of King Rikk, and had a twin sister named Ailsa, who gave her life to save the Prince from Krogg the Red. This Krogg's counterpart was the bestial man who later battled both Supermen after having been exiled in Limbo for centuries.
One, or both versions of, Viking Prince was frozen in ice and thawed out in the 20th century wherein he aided Sgt. Rock in overturning Nazi schemes in Europe before finally perishing. However, that particular Prince was different in appearance, and so may not be the same man. But then there is the question... which of these Jon's were from which Earth? Difficult to say, but regardless, both bore the same mark in history as the premier protectors of their past century long ago.