What is the mileage of your X5?It is very rare both sensors to be bad,if not to say that it never happens.If,for example, there is a lean condition due to a vacuum leak,the ECU may throw a O2 sensor related code,but there is a difference between code pointing to the 02 sensor and bad 02 sensor.In the original BMW programs there are tests to be run to evaluate the condition of the sensors. If the Indy wants to replace both the technician has to show you the reason to be replaced or take responsibility if after spending $800 the problem persists.