Complex sentence: In December, he hosted and performed at radio-sponsored Jingle Balls in New York, Miami and Tampa; and headlined the Gridlock New Year's Eve 2010 festival at Paramount Studios in Los Angeles, which benefited the American Red Cross.
Split into simpler sentences: In December, he hosted and performed at radio-sponsored Jingle Balls in New York, Miami and Tampa. To ring in the New Year, he headlined the Gridlock New Year's Eve 2010 festival at Paramount Studios in Los Angeles, which benefited the American Red Cross.