This is extremely risky.
If OP no shows on his BA ticket, it will be cancelled and it will retain whatever value if has under the fare rules for that ticket. While on a single ticket (not PNR, but ticket), MN as the late delivering carrier would be obligated to rebook (and BA would know that OP was a misconnecting passenger), MN would have to be willing to go with its proverbial hat in hand to BA to have BA reinstate the cancelled ticket. Alternatively, OP would have to go hat in hand to BA to seek reinstatement.
That might all be done without cost, with a penalty fee or with an entirely new ticket, all depending on the BA fare conditions and the whim of someone at BA in JNB.
Anecdotally, people get lucky and they get unlucky. If you rely on travel insurance, be certain that it covers missed flights on separate tickets. Many policies do not.