For once the WTS has some standing with this. Basically, revised secular history dates the 20th of Artaxerxes in 445 BCE. The WTS needs to have the 20th of Artaxerxes occur 10 years earlier in 455 BCE to fulfill their interpretation of the '70 weeks" prophecy and have everything still work out for 1914. So they did a little research and, indeed, discovered that archaeology from Persepolis confirms a co-rulership between Xerxes and Darius I! They, in turn, use this to claim there was a 10-year corulership so that they can date year 20th of Artaxerxes to 455 BCE to fulfill the prophecy.
To align with the timeline again, they add 10 years to the rule of Artaxerxes I, making his rule 51 years instead of 41 years. Amazingly, they have two extant ancient documents dated to year 51 of Artaxerxes. COJ discusses this, but what can you do; the texts exist. They have an option to believe they are more reliable than others that clearly date the reign of Artaxerxes I at 41 years.
BUT... none of this really matters. The Bible at Ezra 6:14,15 clearly limits the rule of Darius I tio just six years and places "Artaxexes" on the throne as his successor. That's because the throne name for Xerxes was "Artaxerxes." Most of the Persian kings used two names so this is not unusual. Except Xerxes, after his defeat in Greece was persuaded to fake his own death and claim "Artaxerxes" was his own son. That's where the problems come in. Xerxes and Artaxerxes were the same king! This is proven at the tombs at Naqshi-Rustam which show that "Artaxerxes" was buried after Darius I, proving he was his successor.
BOTTOM LINE: The bottom line is that Xerxes and Artaxerxes were the same king and his 41-year rule does begin before the end of the reign of Darius I, and thus there indeed was a co-rulership between Xerxes and Darius not recognized by revised secular chronology. The co-rulership was only 4 years though.
Bottom line is that Ezra 6:14,15 in and of itself requires a reduction of 51 years of fake history during the Persian Period. Also, the prophecy about rebuilding Jerusalem was indeed fulfilled by Cyrus in 455 BCE, which is the date you must begin his rule once you remove all the fake Persian years (i.e. -30 from Darius I and Artaxerxes II, -21 from Xerxes who was Artaxerxes, and -1 for a co-rulership vs. sole rulership year for Cambyses.)
The WTS, instead of following strict Biblical history has trusted some secular references but the wrong ones; the Persians revised the entire timeline, removing 26 years from the NB Period and adding some 82 years to the Persian Period down to the time of Artaxerxes III, when the timeline is back in sync.
LS