How to remove the low coherence zone? Is it necessary that the cut-off number will the same for iw1and iw2. While we have the different deformation rate?
you mean if you should apply the same coherence threshold for different sub-swaths?
It would make sense too me to, but maybe a screenshot of your data would help to clarify your question.
have you tried to apply TOPS merge before the terrain correction so that both swath are in the same product?