You can wire the motors in series, and you can also just use a string of series diode pairs to reduce the voltage to the motors. While wiring in series works, there are some downsides. One issue is that if one motor loses traction, the other truck will stall as the free spinning motor will draw much less current. With parallel wiring, that doesn't happen.
Series diode pairs serves to drop the motor voltage while still having additional voltage to the sound board.
