The career trajectory of a dancer is so short that, from a very early age, talented specimens are encouraged to devote themselves heart and soul to their art, and not to waste a minute on frivolities like, say, higher education. The result is that many dancers, who can expect to retire their toe shoes before they turn 40, are simply not qualified to do anything else once they do retire. Two prominent New York ballet companies have been looking to change that unfortunate equation, offering its young dancers the chance to take classes at conveniently located colleges, even as they continue their rigorous dance studies.