If you're travelling with another passenger, it's wise to leave all the carryons with him/ her while checking in at JNB. Your best bet is to use the kiosks, and completely bypass the checkin counters. The kiosks are a little particular btw.

I once had an agent, not even in the correct section (checkin at JNB is divided by sections depending on destination), shout after me (I was simply walking past) that my bag looked too heavy and to please come to her to weigh it. You do not want to check luggage in South Africa.