I've literally just came off a 90 minutes phone call but I did get two F (well, A) seats to/from JNB over Christmas, so overall I'd call that a success. The agent at first told me that it wasn't even possible to combine 241 and GUF2. I pushed on the point and after putting me on hold told me it was possible. Then same thing happened as no redemption availability. Overall the agent was very helpful, I guess there are just so few people overall doing this that they probably have never seen it and they default to 'not possible'.

Only thing I'd say is try to sound like you know what you are saying and most likely that will convince them to check their documentation (that's what happened in my case) and proceed. The booking itself was a painfully slow process as apparently the "automated system" was not working so they had to add us "manually" to both flights. I am not sure what that mean but that I was told.

Good luck, HUACA your way through when you have an opening of at least a couple of hours to stay on the safe side!