In the best of all possible worlds, dance and live music are inseparable companions, an ideal that has become a tradition at Tanglewood, where every summer about this time the Mark Morris Dance Group fuses its considerable talents with those of the newly-arrived fellows of the Tanglewood Music Center. And, as a bonus on Tuesday evening (and repeated Wednesday) in Seiji Ozawa Hall, the Morris dancers also enjoyed the good company of an old friend, Yo-Yo Ma. The genial and gifted cellist was on hand for an encore performance of Morris' popular "Falling Down Stairs," an original collaboration between Ma and Morris explored here in 2003, the first year the festival welcomed the choreographer.