Because of the tandem phase of S3 A/B it is possible to harmonise both sensors.This has been documented in two papers:
OLCI A/B Tandem Phase Analysis, Part 1: Level 1 Homogenisation and Harmonisation
OLCI A/B Tandem Phase Analysis, Part 2: Benefits of Sensors Harmonisation for Level 2 Products
BC experimentally implemented this as GPF graph. This shall be now turned into a usable operator.