The efficiency of two Batmen was not merely one plus one equaling two. They quickly finished searching through the vast majority of the historical texts and really did uncover some problems.
First, both Batmen focused on the historical fragments they had seen before entering Dr. Sohip's room. Combined with certain details described in the books, they basically determined that those scenes came from sometime between ancient history and the Roman Empire.
But the problem was that some things had appeared that should not have existed in human history, such as ravens with glowing red eyes, or those spacecraft and weapons.
This meant there really were other things hidden within history. Or rather, if one turned humanity's history books over, those things once regarded as myths and legends were very likely real.
Of course, even from Wonder Woman's own account, Batman knew that gods existed on Earth. But Wonder Woman was, after all, a descendant of gods herself, and her account of the Olympus pantheon came more from a god's perspective.
Yet through his research into the history books, Batman realized that these gods had enormous numbers of followers during certain periods of Earth's history. Some of those followers might even have been nations they knew well.
From that perspective, it made little sense for Barbatos to have only the Court of Owls as followers. He could not possibly have arrived on Earth only after the founding of the United States. If he had come earlier, the followers he left behind were more likely to be in Europe or Asia, the two continents that had been densely populated in ancient times.
Realizing that the Court of Owls might not be Barbatos's only followers, Batman began searching for ancient beliefs inclined toward darkness. Similar concepts and descriptions existed in almost every pantheon;