The second wire goes directly from the other terminal screw on both buttons to the terminal screws on the bell, (Fig. 4).
A third wire connects to the left terminal screw on the bell and then to the other terminal screw on the transformer, (Fig. 4).
The disadvantage to an arrangement of this type is that the person hearing the bell never knows whether the caller is at the front or back door. The pushing of either the front or back door buttons activates the same bell and makes the same sound.