Defender Cupido and striker Ngwenya have both penned long-term deals with The Citizens, who have now completed six new signings ahead of the 2018/19 season.
The 20-year-old duo graduated from the Diambers academy and have represented South Africa at various levels.
Cupido was notably with the Under-17 side at the 2015 FIFA U-17 World Cup, while Ngwenya was part of the U-19 side that featured at the Durban U-19 International Tournament in 2015.
City have confirmed that Cupido – a former Bidvest Wits defender who joins from French side AC Ajaccio – will link up with the first team immediately, while Ngwenya will start out at the club's reserve team.
Benni McCarthy's outfit have also welcomed midfielders Riyaad Norodien and Gift Links, defender Kouassi Kouadja and striker Siphelele Mthembu thus far.