How dies flanking work?
702.24. Flanking
702.24a Flanking is a triggered ability that triggers during the declare blockers step. (See rule 509, "Declare Blockers Step.") "Flanking" means "Whenever this creature becomes blocked by a creature without flanking, the blocking creature gets -1/-1 until end of turn."
702.24b If a creature has multiple instances of flanking, each triggers separately.