According to Bimetallists in the USA, Gold Monometallism Produces FX Instability between Countries on Different Metallic Standards

Another argument advanced by bimetallists against the substitution of gold monometallism for bimetallism was that it had broken down the so-called nexus between countries on different metallic money standards. As long as some countries were on a bimetallic standard, with the unlimited coinage of both gold and silver at a fixed mint ratio, the fluctuation would be very slight in the gold price of silver in gold-standard countries and in the silver price of gold in silver-standard countries. Foreign exchange rates, therefore, among countries on all three standards--bimetallic, gold, and silver--would be stable, as they were prior to 1873.

Since discontinuance of bimetallism, it was said, all this had been changed. The nexus between gold and silver was broken and each metal had gone its own way. There was thenceforth no limit to the possible variation in exchange rates between a gold-standard country and a silver-standard country. This brought a large new element of risk and speculation in foreign trade between countries on different metallic standards. It was an obstacle to the development of trade between gold- and silver-standard countries, as well as to the flow of funds for investment between such countries.

--Edwin Walter Kemmerer, Gold and the Gold Standard: The Story of Gold Money, Past, Present and Future (New York: McGraw-Hill Book Company, 1944), 91-92.