Checked the 1941 and 1943 Electoral Roll but no Elmer listed anywhere in SA.
Also the SA Directory 1924 - no Elmer but an Alf Thompson, assistant at Two Wells, which may connect with the letter you mentioned. No further info.
What was the false name used when enlisting in WW1. ?
If they were involved in horses, the horses could have been stabled in the Two Wells area yet the ELMER family could have lived elsewhere. There were (and still are) stables, trotting tracks and race courses in that area.
Regards,
Sashar