The Springboks have, however, not brought in lock cover despite losing Lood de Jager and Franco Mostert to red cards in the last two weeks, though the latter must still face his disciplinary hearing for his sending-off against Italy.
De Jager was suspended for four weeks following his red card against France.
"This is a longer tour than usual, and we have two big matches lined up against Ireland and Wales, which prompted the decision to call up Bongi and Ntuthuko," coach Rassie Erasmus said in a statement on Sunday.
"Both players have done the job for us on the international stage, so we are excited to have them in the group.
"Several players will not be eligible for selection for the final Test on tour against Wales, as the match falls outside of the international window, so it makes sense to call up the players now to get back into the swing of things with us at training this week, while at the same time increasing the depth we have within the squad for our next challenge against Ireland."
The top-ranked Springboks have three wins so far on tour after defeating Japan (61-7), France (32-17) and Italy (32-14).