Stick two trombones in front of a rhythm section and most people imagine one of two scenarios: the exacting give-and-take of J.J. Johnson and Kai Winding, the bop-era ‘bone men whose onstage jousting sounded less like a pitched battle than a sophisticated conversation, or the longest four minutes of a high school stage-band concert. Because the trombone produces different notes with the help of an infinitely variable slide–as opposed to a trumpet piston, a piano hammer, or a saxophone key, all of which work pretty much like two-position switches–it’s a challenge just to play in tune. And even professional trombonists risk wearing out their welcome by pairing up, since few have the technique and imagination to trade licks for a whole set, much less a whole night. But when it works, it really works, and that’s the case with the partnership of New York bandleader John Fedchock and Chicago soloist and educator Paul McKee–friendly rivals from their days together in Woody Herman’s band of the mid-80s. Fedchock, who’s recently worked mostly in front of his tough-minded New York Big Band, plays with a dark, virile, sharply etched tone that erupts into split-note rips in its upper registers, and he’s as well organized and aware of texture in his articulate solos as he is in his terrific arrangements. McKee, on the other hand, tends to shoot from the hip, and his complicated postbop figures have a lighter, smearier sound. He makes full use of the trombone’s slipperiness to slur through some of his trickier phrases, but still manages to center each note; his lines convey plenty of motion without losing their focus. Before they teamed up last year, the Green Mill had never hosted two trombonists on the same bill. But Fedchock and McKee blew the roof off the old joint, and this time the set should be even more explosive. Their quintet features Karl Montzka on piano, Kelly Sill on bass, and the splendid yet often overlooked Tim Davis on the traps: no drummer in the city listens more carefully to the music in front of him. Friday, 9 PM, and Saturday, 8 PM, Green Mill, 4802 N. Broadway; 773-878-5552. NEIL TESSER