C ollaboration is an artform in itself, as Southbank Centre’s Multitudes festival has demonstrated over two weeks of sometimes divisive but never-less-than stimulating creative cross-fertilisation. This final concert, fusing wildly contrasting disciplines, was among the most nourishing, a performance in which each partner had immersed themselves in the working practices of the others. The palpable sense of collegiality and mutual respect was as heartwarming as the music-making. Thick & Tight’s gracefully integrated movement in Papillons. Photograph: Rosie Powell The subject was butterflies, nature’s metamorphic miracles, whose complex physiological processes and unerring sense of purpose culminate in an eruption of kaleidoscopic colours.…