At 1910 Pacific time, a Beech 76 suffered a collapsed landing gear during a practice single-engine go-around at Petaluma Airport. The flight instructor, the commercial-licensed multiengine student, and one additional passenger were not injured. The instructor said they were performing a single-engine VOR approach in VFR conditions when he decided to have the student do a single-engine go-around. However, he issued the go-around instruction too late and the airplane touched down with full power on the left engine, causing the pilots to lose control. The instructor said neither he nor the student were making aggressive enough control inputs to correct the situation in the early stages and he may have waited too long before assuming control.