why did populists want the united states to move from from a currency backed by gold to one backed by both gold and silver

A bimetallic standard allows the value of paper currency to be linked to the value of metals. In this case, the value of gold and silver. Populists felt that, unlike, the Gold standard which was favored by Grover Cleveland and William McKinley, the bimetallic standard would increase the supply of money, stabilize prices and facilitate exchange rates.