you can look this up on gendatabase.com - but you will need to pay - the data comes from the Population records
CORNELIUS FRANCOIS CHRISTIAAN DU PLESSIS 1946-05-29 MARRIAGE DATABASE
I think this is their marriage date - but without a valid subscription I cannot confirm wether or not they have a location for the marriage
This may be Naomi's birth Date
NAOMI DU PLESSIS VILJOEN 1927-05-29 DEATHS DATABASE if you pay you will find her Death Date and location
These records could be Cornelius
CORNELIUS FRANCOIS CHRISTIAAN DU PLESSIS 1923-04-18 DEATHS DATABASE View Record
CORNELIUS FRANCOIS CHRISTIAAN DU PLESSIS 2005-03-26 MARRIAGE DATABASE Signup to View
CORNELIUS FRANCOIS CHRISTIAAN DU PLESSIS 13 Mar 2005 MASETI DATABASE Signup to View
You can sign up for 1 day for $9.95 or a week for $12.95.
The estate number DU PLESSIS Cornelius Francois Christiaan 1923-04-18 2005-03-13 Cape Town 5971/2005
Unfortunately this one is not on-line as yest - but if you keep an eye open on familysearch.org it must be coming soon as they have put a place holder on this collection
https://www.familysearch.org/search/catalog/2156332?availability=Family%20History%20Library - once a camera appears next to the film holding the file number you will be able to access it.
The estate file may give you his parents names, children and confirm his spouses as well as his location of birth and death.
Here is Naomi's estate number and a link to the records
DU PLESSIS Naomi 1999-01-30 Cape Town 1828/99
https://www.familysearch.org/ark:/61903/3:1:3Q9M-CSQK-19SL-5?i=390&cat=2156332Gives her birth location as Bloemfontein and their marriage location as Langlaagte - unfortunatelly does not list her parents but you could look through Bloemfontein Baptisms for
Regards
Gillian