The owner/builder of the experimental amateur-built, tailwheel-equipped biplane was seated in the front left seat, with another pilot seated in the right seat, and a passenger in the rear seat. The intent of the flight was to prepare both pilots for a flight review, with both trading off flight duties as necessary. They planned to initially perform touch-and-go landings at their home base airport, with the pilot in the right seat acting as pilot-in-command. The first takeoff and landing was uneventful, but the airport was busy, so they diverted to the accident airport to practice further. The landing approach was normal, with the airplane touching down slightly to the right of centerline and then bouncing. The pilot made corrective control inputs, but the airplane did not fully respond, bouncing again on the runway. He applied forward elevator pressure and the airplane landed a third time, this time remaining on the ground. The airplane began to drift to the right, but would not respond to the pilot's rudder inputs. The owner/builder then applied left rudder, but the airplane continued to drift towards a runway sign. As they passed to the right of the sign the airplane pitched down violently, and nosed over. The airplane sustained substantial damage during the accident sequence to the left upper wing and the vertical stabilizer. Postaccident examination of the airplane revealed no mechanical malfunctions or failures which would have precluded normal operation. The owner stated that this was the first flight since he had completed the airplane's conditional inspection, and no flight control malfunctions were discovered at that time. He subsequently stated that he did not find any mechanical malfunctions or failures during a post-accident examination. He further surmised that the design of the braking system was such that simultaneous application of the left and right side brake pedals by both pilots may have resulted in excessive brake pressure, and this may have been the reason the airplane nosed over so unexpectedly.