Here are a few shots of the tree show by DBRP.
The two wooden forms in the foreground were feed backing resonators/ drums which contained a microphone and a speaker each. They are hybrid bazooka speakers much like ones used in autos. The third form, the tower, in the back contained an amp and speakers. The microphone ran from one "drum" through pedal effects to the amp then to the opposite speaker in the other "drum".
Two performers where stationed outside the gallery each with one "drum" 100 yards apart. What one played on one "drum" was heard on the opposite drum heard by the other performer. Both performers were heard by people in the gallery through the speakers housed in the tower.