I have found by inadvertance a tablet that confirm that there never had a co-regency between Darius I and Xerxes in a french's website "achemenet.net".
It is important, because i have never seen before a double-dated tablet between this two kings, and Rolf Furuli, as Gerard Gertoux a french JW, has tried (in accordance with the book "Insight" of the Watchtower) to prove a co-regency between the two king in the final goal to have the 20th years of the King Artaxerxes in -455. This date is important for the Jw's calculation of the prophecy of Daniel 9, concerning the 70 weeks.
If there is no co-regency so the reign of Artaxerxes has really start in -465 (and note -475 like the JW's theory) and the interpretation of the 70 weeks of Daniel by JW is false.
Here is the translation in english of this tablet, that Carl Olof Jonnson has provided to me, after i have contacted him concerning this tablet