Charles Frodsham, An unusual two-day boxed marine chronometer with two balances on one arbor and 24 hours dial, no. 1707, circa 1840

brass full plate movement, later one-piece balance cock, diamond endstone, two bi-metallic compensation balances set at 90 degrees to each other, blued steel helical spring, Earnshaw's spring detent escapement, fusee with Harrison's maintaining power, silvered 24 hours dial, arabic numerals, subsidiary seconds, sector for up-and-down, signed and numbered, blued steel fleur-de-lys hands, gimbaled brass bowl, ratcheting key, three-tier brass bound mahogany box with ivory signed plaque, accompanied by a copy of Randall, The Time Museum Catalogue of Marine Chronometers.

Time Museum Inventory No. 685

Randall, cat. no. 67

The reasoning behind the double balance remains obscure, certainly it would have made for the ability to adjust the temperature error finely but since it remains rare to find examples, one can only assume it did not make the chronometer sufficiently more accurate to justify the expense. 

For another double balance chronometer by Charles Frodsham, no. 2006 dated 1852, see Asprey & Co. Ltd., The Catalogue of An Exhibition of Marine Chronometers, London, 1972, described and illustrated pp. 13-14.