Any space-born SAR system has both ascending and descending orbits, for which acquisitions cannot be at exactly the same time. Selecting insar pairs from orbits close in time is a common practice, assuming no much change happened in between.
Please note that asc & desc insar measurements are done along different line-of-sight (LOS) and cannot be combined simply using averages. You need to geometrically combine considering acquisition geometries. There should be several related publications in literature.