“He was once asked what a friend is, and his answer was, "One soul abiding in two bodies."”

Letter IX.
Letters

Platonic Questions, viii, 4
Bartlett's Familiar Quotations, 10th ed. (1919)

Borejza, Tomasz (January 2018): Trochę bakterii nie zaszkodzi https://www.tygodnikprzeglad.pl/troche-bakterii-zaszkodzi/. Przegląd (4/2018): pp. 54–55.

Life of the Duke of Alva (1642). Compare: "A fiery soul, which, working out its way, Fretted the pigmy-body to decay, And o'er-inform'd the tenement of clay", John Dryden, Absalom and Achitophel, part i. line 156.

Source: The Candy Shop War

Did not appear in Saturday Evening Post story, but quoted in Einstein: His Life and Universe http://books.google.com/books?id=dJMpQagbz_gC&lpg=PP1&pg=PA387#v=onepage&q&f=false by Walter Isaacson, p. 387, in the section discussing Viereck's interview.
1920s, Viereck interview (1929)

Source: The Souls of Black Folk (1903), Ch. I: Of Our Spiritual Strivings
Context: After the Egyptian and Indian, the Greek and Roman, the Teuton and Mongolian, the Negro is a sort of seventh son, born with a veil, and gifted with second-sight in this American world, — a world which yields him no true self-consciousness, but only lets him see himself through the revelation of the other world. It is a peculiar sensation, this double-consciousness, this sense of always looking at one's self through the eyes of others, of measuring one's soul by the tape of a world that looks on in amused contempt and pity. One ever feels his twoness, — an American, a Negro; two warring souls, two thoughts, two unreconciled strivings; two warring ideals in one dark body, whose dogged strength alone keeps it from being torn asunder.The history of the American Negro is the history of this strife, — this longing to attain self-conscious manhood, to merge his double self into a better and truer self.

Man's Search for Meaning (1946; 1959; 1984)