Phase noise measurement with IQ mixer
The mixer output should not be sensitive to small variations in amplitude on the LO path. So, any amplitude error in the measurement will be dominated by the amplitude error from source SG2.  In general, the amplitude error of a good signal generator is very low.  So, the error you measure is almost entirely a combination of the phase errors from both sources.

As a check, you can measure the amplitude error by itself using a single signal generator.  Split the output and apply the signal to both the LO and RF inputs.