William Cox had two wives (that we know of). Many family trees on the internet merge these two women as one, but we have documentation that common sense shows they are two people.
In early land records, she signs her name using just Alce Cox. If she went by another name or both, it seems like she would use both but she only appears as "Alce" or "Alice."
It is established that Mary Langford was a widow. She is mentioned in her first husband's Henry Langford's will and probate records. Henry Langford died in 1794. A warrant for the appraisement of his property was issued on October 7, 1794. Mary Langford was the administrator of her husband's estate. The land was purchased by Jacob Black, Samuel Pyle and Robert Langford (Henry's brother). Those Black and Pyle names are familiar and intermarried into the Cox family. Source: General Index to Probate Court and Estate Papers, Greenville County, SC// Langford, Henry, Apt#4, File#275, Year Filed 1794 (starts page 324 in pdf)
As part of those records there is even this statement from October 1806:
"On the appearance of William Cox, who married the widow and administrator of Henry Langford deceased agreeable to citation and his wife and say that the whole of the personal estate of said Henry Langford hath been expended in supporting the widow and children of said deceased and that there is yet in their hands a tract of land which they hereby agree and oblige themselves to give up to the heirs of said Henry Langford without any objection or keeping back any part thereof."
Also, if she is also the same Mary Cox who died May 9, 1844 at age 95, which seems very likely, she would not have matched ages for the children of William. She would have been born around 1749. She was younger than her second husband William.
Death details from "Marriage and Death Notices from the Up-Country Of South Carolina as taken from Greenville Newspapers 1826-1863" -- On May 24, 1844: "Died at the residence of Abraham Cox, of this district, on Sunday, the 9th inst., Mrs. Mary Cox, aged 95 years." This would be her stepson or stepgrandson, if it's the same Mary.
More about Henry Langford (although I haven't fact-checked this info and apparently there are two men with similar names that get conflated into one so take with grain of salt.