Just following on from what Stewie said you will have to recheck your bag thus meaning no interlining. This is due to you having to clear customs at JNB , they have a really neat setup for all international arriving paxs into JNB who are connecting onto SAA. When leaving the immigration hall you will see a load of SAA reconnection counters , so no need to work your way through the crowd and go to the really busy SAA domestic check in counters.