Tenors Jian Wei Han and Alfio joined cultural and vocal forces to sing popular Italian songs at the first annual Marco Polo Day last Sunday. They were celebrating the designation of the Chinatown Little Italy Historic District, which was just added to the State Register of Historic Places. A national designation is expected shortly. The duet of Han, from China, and Alfio, an Italian-Australian who records under one name, offered a concrete reminder of how the two cultures mingled Downtown over the past century and a half. Victor Papa, president of the Two Bridges Neighborhood Council, who pushed for the designation, helped lead the ceremonies Sunday at SPQR Restaurant on Mulberry St. Borough President Scott Stringer presented a proclamation officially naming Oct. 18 Marco Polo Day.