It’s not often that two of the city’s largest arts organizations decide to work together as equal partners on a project, but it’s happening next weekend with the Louisville Ballet and the Louisville Orchestra. Their “Spring Collaboration” program will feature both new and established dance pieces, with music performed live by the orchestra, which will be onstage with the dancers at some points. The orchestra’s music director, Teddy Abrams, also composed a new piece of music for the occasion, and he’ll be conducting as well. The collaboration came out of the friendship between Abrams and the ballet’s artistic director, Robert Curran, a project which Curran says was “inevitable.”