Guide to Connecting at JNB
My 1:50 connection for CPT-JNB on SA and JNB-CAI on MS on the same PNR was more than enough time. My bags were checked through and all BPs given in CPT so it was just the time to get from one gate to the other. The total walk time from Domestic arrival to Int'l departure gate shouldn't be more than 15 minutes at a healthy adult pace so I believe 65 minutes will be fine.
What time of day/week is the trip? The biggest time risk will be getting through security and border control as you pass between terminals. It was pretty empty on the Sat evening I connected but the posts were understaffed so it still took about 15-20 minutes. However, there appeared to be enough lanes to handle substantially more traffic so hopefully they balance staff to expected passenger numbers to keep the lines moving.