I have received 2 TerraSAR-X Staring Spotlight images taken in an interval of 11 days and in MGD format. One is HH, the other VV, both are the same location.
Are there any processing steps I can perform to improve image quality outside Single Product Speckle Filtering?
You could, after coregistration, apply a multi-temporal speckle filter. But it will probably not be very effective with only two images (which are even from two different polarizations).
What I like to do with my TSX ST data is calculating an average image. You will see quality increase with every image you add.
I coregistered both images with SNAP default options. The resulting stack is 2 bands, one exactly the same as the master (hh) and the other (VV) is all black (0 value). Am I doing something wrong?
you can increase the windows size of the cross correlation and the number of GCPs. Also check the box for the residual report so you can see how many GCPs were left at the end.
Try the DEM-based coregistration instead. It uses a downloaded DEM (preferably SRTM 1ArcSec) to simulate an image which is then matched against both radar images to find their relative shift.