It often happens, that Sentinel doesn’t take pictures of an exactly same place. One-two burst difference may take place.
I am wondering, if two pictures overlap covers area, that I’m interested in, what is the maximum allowed image shift, so that co-registration can be trusted?
I assume you are talking about azimuth shifts. That is common for Sentinel and it’s not a problem.
You should not have any coregistration issues.
Moreover, you can put consecutive images in the SLC folder: Yuxiao developed a simple algorithm to merge them, and you can treat them as a whole image.