Percy and Simpson, who teamed to win the two-hand Star class in Beijing four years ago, settled for silver ahead of Brazilians Robert Scheidt and Bruno Prada.
Loof and Salminen went into the medal race, the final race including the top 10 teams, in third place behind the British and Brazilian duos.
Although he was unable to capture gold, Percy added a silver medal to a resume that also includes gold in the Finn class in 2000 as well as his Star triumph with Simpson four years ago.