Given that you are connecting Int to Domestic, you will have to collect your bags in JNB anyway and recheck them, irrespective of whether or not your bags are checked through.
Also, you will have to clear immigration in JNB and head to the domestic terminal, I would advise on checking in online and printing your Mango BP in advance, otherwise you can either get them at the check-in counter or using the automated kiosks.
Bear in mind that queues of 1hr in immigration is not unheard of, so for a 2hr connection I wouldn't recommend on having checked luggage as bag drop/check-in closes 45min before the flight scheduled time, so make sure you have OLCI'd, otherwise I would advise a 3hr connection.