wiki comments that were kind of a quick synopsis....
Carl Olof Jonsson, a former Witness, states: [48]
Either the Watch Tower Society conceals the real facts about these two tablets, or they have done very poor research on the matter. The first tablet, designated "A. 124" by Thompson in his Catalogue from 1927, is not dated in the accession-year of Xerxes (486/485), as Thompson indicated. This was a copying error by Thompson. The tablet is actually dated in the first year of Xerxes (485/484 BC). This was pointed out as far back as in 1941 by George G. Cameron in The American Journal of Semitic Languages and Literature, Vol. LVIII, p. 320, ftn. 33. Thus there was no "overlapping" of the two reigns.
The second tablet, "VAT 4397", published as No. 634 by M. San Nicolo and A. Ungnad in their work from 1934, was dated by them to the fifth month ("Ab"). It should be noted, however, that the authors put a question mark after the month name. The sign of the month on the tablet is damaged and may be reconstructed in several ways. In the more recent work by Parker and Dubberstein, Babylonian Chronology, published in 1956, where the same tablet is designated "VAS VI 177", the authors point out that the tablet "has the month sign damaged. It might be IX [9] but more probably is XII [12]." (Page 17) The original guess by Nicolo and Ungnad is dropped altogether. As Darius died in the 7th month, a tablet dated to the 9th or 12th month in the accession-year of his successor is quite all right."
Rolf Furuli, one of Jehovah's Witnesses, reviewed data from about 7,000 business tablets from the New Babylonian Empire, concluding that just over half of the evidence that Parker and Dubberstein uses has no real value. [49] He stated that "A comparison of 1450 cuneiform tablets dated in the reigns of the Persian kings reveals tablets for most of the kings that contradict P&D's scheme." He goes on to say: [50]
The most trustworthy cuneiform evidence consists of dated business tablets. ... A comparison of these tablets suggests a different chronology compared with the one advocated by P&D. The business tablets demand that Bardiya (Gaumata, who probably is the "Artaxerxes" of Ezra 4:23) ruled for 18 months between Cambyses and Darius I. Thus the accession year of Darius I is pushed one year forward. There are strong reasons to believe that Xerxes was co-regent with Darius I for 11 years. (Evidence: 1) Tablets are dated to the accession year of Xerxes before the last tablets of the 36th year of Darius I, 2) drawings and inscriptions make the two equal and give them the same titles, 3) different titles used by Xerxes after his 11th year, 4) the pattern of intercalary months is the same in the last 11 years of Darius and the first 11 years of Xerxes). The reign of Xerxes is pushed back 10 years (11 years of the co-regency minus the 1 year of Bardiya). Thus the accession year of Artaxerxes I is taken as 475 B.C.E., ten years before 465, which is advocated by P&D. This means that the 20th year of Artaxerxes (Neh. 2:1) is 455 B.C.E. The business tablets may indicate that Artaxerxes I ruled 51 years or even through his 51st year and a few days into his 52nd year. Thus the reign of Darius II is pushed forward one year (Xerxes II/Sogdianos may have ruled a short time between Artaxerxes I and Darius II).