The opera star Anthony Roth Costanzo and the cabaret diva Justin Vivian Bond hail from opposite sides of the tracks, vocally speaking. But when the two unite in their stage show, "Only an Octave Apart," something fabulous emerges. This performance from, as Bond puts it, Costanzo's "tiny apartment," with his "tiny furniture" and his "tiny friends," gives us a glimpse of the show that opened at St. Ann's Warehouse in Brooklyn last fall and landed at the New York Philharmonic earlier this year.