Question

which constitutional amendment requires electors in the electoral college to cast separate votes for president and vice president

Answer 1

The constitutional amendment that requires electors to cast separate votes for president and vice president is THE TWELFTH {12}  AMENDMENT. 
The twelfth amendment was enacted in 1804 and it requires that electors should case separate ballot for the president and the vice president. If it now happens than no candidate receives a majority, then the house can choose the top three candidates for the president and the top two candidates for the vice president.