Does the flanking provided by Slivers stack?
Each instance of Flanking triggers separately. After the stack resolves, creatures blocking will have -2/-2 until end of turn.

The Rules:


  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.