Please have a look in the documentation every now and then 
Sigma0 is the standard output.
Beta0 and Gamma0 are just optional (the first is a lesser, the latter a higher level of calibration)
I your case, calibration to Sigma0 is no longer possible because the image geometry was already altered during the coregistration process.
Radiometric calibration only affects the intensity is therefore not required for InSAR. If you want two images of Sigma0 in one stack, first calibrate them and then perform coregistration.