My objetive is to merge the two products
I just pre processing the SAR data and the optical data like your instructions with SNAP, but when I create a Stack I receive that error message when I upload both products:
I make this steps: Apply prexise orbit file > S1 Remove GRD Border Noise > S1 Thermal Noise Removal > Calibrate > Multilooking > Speckle Filterin > Terrain correction. at the end I get this image:
then I have applied the Coregistration, choosing as products the image s1 with an image S2 of the same date. I have obtained this image as the final image:
You can play with the contrasts of red and green in the color manipulation tab to enhance the different color tones. Currently, the image tends to be greenish.
It is advisable to convert the Sentinel-1 backscatter to dB scale for a more gaussian distribution of grey values.