At the crux of Walt Whitman’s “Crossing Brooklyn Ferry” lies the question: “What is it, then, between us?” In Crossing, 27-year-old composer and conductor Matthew Aucoin explodes Whitman’s “us” to operatic proportions, taking inspiration from the diary the poet kept while volunteering as a Civil War nurse. Stunningly rendered by pioneering, Boston-based orchestra collective A Far Cry and American Repertory Theater artistic director Diane Paulus (Waitress, Pippin), Aucoin’s taut chamber opera imagines Whitman, played by baritone Rod Gilfry (the loser, 2016 Next Wave; Anna Nicole, 2013 Next Wave), drawn deep into a fraught, sensual relationship with wounded young soldier John Wormley (tenor Alexander Lewis). In the shadows of a DC sick ward, haunted by the dreams and nightmares of its inmates, the two men grapple with questions that threaten to tear each other—and their country—apart.