Trewartha and Andrewartha is a Cornish family name (and placename, Dexter).
According to the Handbook of Cornish Names by G Pawsley White, "Trewartha" is a Cornish name meaning "Upper Farm" or "Upper Homestead".
CORNISH NAMES by Dr T. F. G. Dexter : Royal Institution of Cornwall : pub 1926 Longmans Green : reprinted 1968 Bradford Barton : says on p15 -
Nouns: "tre" a "town", feminine, (maybe a hamlet or house, also Brythonic Celt Welsh "tref", about 4-500AD). Adjectives: "Wartha" : "upper" (maybe higher or greater or on a hill) cf. "Wollas" : "lower" ( maybe smaller or lesser or in a valley, of the two).
"Trewartha" page 25. "Andrewartha" page 60.
Definite Article: "An", (Brythonic Celt Cornish language)p18, is used as : of the, in the, on the, at the, in place names. [JEA 8.2009]
Tre = Above, Upper (x) - - Wartha = Farm, Homestead (x)
As An is Cornish for The, and Dre is mutated from the Cornish "Tre" Andrewartha means "The Upper Farm" or "The Upper Homestead" and it is generally accepted that both names have the same meaning.