Richard hello, I think we're talking about 2 cousins - both named James Allen and both married Chastain sisters- Martha and Elizabeth. Mine married Elizabeth and William Allens son James married Martha. This is according to the Denton Family home page on the internet. By the way William's brother was Reuben Allen. I did receive some information on this from an Elizabeth Allen in Virginia who confirmed the above information.
I do have info on that family. A note here I have Mary Barnes as your William's wife. But have some info listing her as Abigail Barnes too. (Perhaps Abigail and Mary were both her correct name?) I have 10 children for them- James (m. Martha Chastain), Hugh, John, Millie, Nancy, Elizabeth, Sallie, Lucinda, Jennie, Temperance (Tempie)
Apparently your James settled around Gilmer County, Georgia, my James settled in White County, but the two family kept the road between White and Gilmer hot as my James Allen's son Samuel D., his daughter Virginia and Matilda all settled in Gilmer County.
You will also find that William was in Old Walton County between 1793 and 1805. He is listed on the Old Walton County page on Georgia Gen Web. (look under Yazoo Land Fraud) The area was about a 12 miles strip of land that is now Rabun Co, Georgia, Transylvania NC and the corner of SC (no notes with me right now) You have murder, false imprisonment, threats and finally war over this small stretch of land.
Both William and Reuben are also mentioned in the Book Wayfarers of Walton by Anita Sams.Intersting reading.
In Rabun county Georgia there are places named after the Allens.