No more diodes are needed. There should already be a series diode in the circuit between the battery and the solar panel. Just wire your added solar panel across the existing panel. Doubling the panels in parallel increases the current capacity to charge the battery quicker. The paralled panels should be of identical voltage output.
Adding another diode in series will cause a drop of 0.6 volts. The series diode prevents the battery from draining charge through the resistance of the solar panel at night when the solar panel is not active.
Adding another diode in series will cause a drop of 0.6 volts. The series diode prevents the battery from draining charge through the resistance of the solar panel at night when the solar panel is not active.