How much the Concorde secondary nozzle contribute to thrust?
This source suggests at subsonic speeds, the secondary reduces drag but doesn't contribute significantly to thrust. 


  At all times below Mach 0.55 – on the ground, take-off and initial
  climb – the buckets are set to 21 deg., forming a ‘jet-pump’ gap at
  the nacelle trailing-edge. The high speed jet exiting from the primary
  nozzle draws-in atmosphere air from above and below the nacelle to
  eradicate a source of base drag that would otherwise exit between the
  nozzles.