Hi Julie
Very difficult to get marriage details as one would have to find them in the church records which aren't online as far as I know. There are some registers for some of the churches at the Cory Library for Historical Research (part of Rhodes University)
http://opac.seals.ac.za:81/search~S1 Now that I have given you this information, I have no idea how you would go about accessing it

I guess the obvious would be to e-mail them.
http://www.ru.ac.za/corylibrary/contactusDepending on how much you are willing to invest in obtaining the date of their marriage, you could engage the services of this person who claims to do South African Churches Research & Genealogical Research. I have never used him myself and found his name on the NAAIRS site.
Bishop Dr. M S Mawelele
P O Box 592
Postnet Suite No. 27 Silverton
PRETORIA
0127
Tel: (012) 801 4897
Fax: (012) 801 4897
Cell: 072 381 3956
E-mail: MSMawelele @ yahoo dot com (omit spaces and replace dot with .)
Regards
Moira