I found it. Pages 305 - 306 of The Fated Sky by Benson Bobrick. Way better than my faded memory of reading.
"The analogy, moreover, is strengthened on the astrological side by the phenomenon of "astral twins". One such famous case involved an English subject and his king. On June 4, 1738, in the parish of St. Martins-in-the-Fields, two boys were born less than a minute apart. One was William Frederick, later crowned George the III, King of England; the other, James Hemmings, an ironmonger's son. widely separated by class, yet bound to a parallel fate, these two men, each in his own social sphere, lived out the edict of his stars. In October 1760, when George III succeeded his father on the throne, thereby fulfilling the purpose to which he was born, Hemmings took over his father's business. Both men were married on September 8, 1761, fathered the same number of children (even, weirdly, the same number of boys and girls), suffered the same accidents, succumbed to the same diseases, and died within less than an hour of each other on Saturday, January 20, 1820.
Another famous prince-and -pauper story yoked together a chimney sweep with the Prince of Wales (later King George IV). E. Oakes-Smith, in
The Shadow Land, or the Seer (1852), wrote:
"Of the career of the Prince of Wales it is unnecessary to speak---his vices, his follies, his perjuries were all royal, and his fellow, the sweep, was not a jot behind him. The broom and scraper were as ill-adapted to the hands of one as the scepter to the hands of the other. The parents of the sweep, tired and ashamed of his profligacy, finally established him as a tallow chandler. On the same date, George IV was put on a royal allowance and both men embarked on separate but similarly notorious careers as gamblers, philanderers and spendthrifts---but on entirely different financial and social levels. the commoner acquired a stable of asses and ran the best donkey races of the day. George IV kept the best blooded ponies and ran the finest horse races in the country. On the day that "Prince George" was kicked in the hip by a donkey, George IV was kicked in the ribs by a horse. Both were injured and incapacitated for the same amount of time. when the Prince of wales lost everything and went bankrupt, so did the commoner "prince". And when all the King's horses were sold by the royal horse seller, the ex-chimney sweep lost his asses under the hammer of the auctioneer.""