In some ways, next weekend’s collaboration between pianist Joyce Yang and Aspen Santa Fe Ballet turns one of the country’s foremost classical music soloists into a second fiddle. But, hey, it was her choice. Yang, who has starred center stage as a guest artist with orchestras across the world, came up with the idea of playing backup for the dancers herself, after seeing the company perform in New York two years ago. She asked its artistic team to create a movement that could be featured during one of her signature concert pieces, Robert Schumann’s “Carnaval.” The request said a lot about Yang’s artistry — that she was willing to sacrifice the spotlight to explore new angles on a well-known masterwork — but also about the dance company’s reputation.