You didn't include any dates, or approximate dates, in your query, so I looked at your message history to see if you gave any additional info in other posts.In one you said William Lafayett was born 1849 and married 1849, so assume one of them is a typo.
Is this your William Lafayette Sandidge?1920 census McKinney, Collin Co, TX:
Sandidge, W. H., hd, 47, md, b TN, TN, TN
" Mary, wf, 45, md, b TX, MO, AR
" ____, dau, 1?, sgl, b TX (can't read her givenname)
" Othel, son, 16, b TX
In the same county is a John H. Sandidge, 50, b TN with wife and dau, I would guess him to be a brother to your William.
I do not see William nor Mary on the 1930 census in TX.But, John is still in Collin Co, TX in 1930.
I don't know where you are, but if you are in TX, I understand many of the public libraries have the TX state wide death index 1903-1940 on microfilm.If not, then contact the TX State Archives in Austin, they do have the film.They can tell you date and county of death for both William and Mary.Their death certificates should show names of parents, and that would give you Mary's maiden name, too.
1860 census Bradley Co, TN:
Dobbs, James, 66, b VA, land $1800/personal $200
" Polly A., 37, b VA (Polly is a nickname for Mary)
Sandridge[sic], Har____y, 17, b TN (name not clear probably the John H. on censuses in TX)
" James M., 14, b TN
" Wm L., 11, b TN (your William Layfayette)
" Nancy C., 9, b TN
" Mary E., 6, b TN
Dobbs, Clarinda, 21, b TN (probably dau from 1st mar of James)
" John E.B., 1, b TN
I would look for marriage records in Bradley Co, TN for Mary/Polly.Check Sistlers multi-volume set of early TN marriages prior to 1860.
Happy Hunting (^_^)