A V-type engine will have a minimum of three. One is right before each catalytic converter, and the downstream one can be in the Y-pipe where the two sides connect. For better monitoring of the converters there is usually two downstream sensors; one right behind each converter.
P0137 Oxygen Sensor Circuit Low Voltage (Bank 1 Sensor 2)
Usually this code sets in response to a wiring problem, not a sensor problem. Bank 1 is the driver's side and sensor 2 is the one after the converter. I don't find a reference to the other code.