“Pergolesi” is a different sort of experiment. For it, Tharp has taken a duet that she choreographed for herself and Mikhail Baryshnikov in 1992, and set it on Robbie Fairchild, a former City Ballet principal and Tony-nominated lead for the Broadway musical “An American in Paris,” and Sara Mearns, a City Ballet principal known for stretching herself far beyond ballet. (During the program, Mearns performs in jazz shoes, pointe shoes and ballet slippers — an athletic tour de force, says Tharp.)”