Some great responses so far.
Looking for four redemption seats in any premium class is going to be a struggle; CPT is one of the most capacity restricted destinations due to bilateral agreements limiting flights. Fares are high, and so is demand for redemption seats. Having said that, I have often seen 8 F and 8 J seats available for redemption on many other routes, even quite close to departure.
Add to this the obvious attractions of sunny South Africa in the middle of the British winter, the lack of significant time difference and both Half Term and Valentine’s Day means seats will be extra hard to come by.
However, you are doing exactly the right thing by looking far in advance, and checking back regularly.
As has been pointed out, you can only use your Amex 2for1 voucher exUK, and on BA operated flights.
BA certainly has not “blocked” that route from redemption.
I can see Club World seats outbound every day except 6, 7, 8 Feb, and return is available on 12 of the 22 currently available days.
NTarrant is spot on to state availability doesn’t hit the online system until 350 days out (355 *sometimes* if you call up), so right now 22 Feb onwards is not offered for booking online.
You may have to be flexible on your dates, class of travel, routing or consider redeeming two seats using your Amex voucher, and purchasing the others.
If you have no flexibility on a high demand route, BA Miles (and indeed any loyalty scheme redemption) is not the way to go.
Two seats are available outbound 10 of the 22 currently available days, return is less good with just 4 of the 22 days having flights.
WT+ is wide open for at least two seats for most days.
If you can find two Club return seats on dates which suit, BOOK THEM NOW using your 2for1 voucher.
Then book two WT+ seats on the same flights, so at least you can travel together.
Oddly there doesn’t seem to be any F availability, but this could change so keep checking back.
There are some other other options, such as:
1. Remember you can mix the class of your booking outbound in WT+ and back in J or F, for instance.
other options, though these will not work using your 2for1 include:
2. Checking availability from different European countries, which can indeed vary from the UK available seats, but this is not for the fainthearted and would involve a bit of research and the use of a tool like expertflyer.
3. Selecting a “stopover” via Johannesburg though this does involve a short sector on BA affiliate Comair (so no 2for1 options) to CPT in a CE cabin; again you can outbound via JNB and/or return via CPT so check both routings.
4. Though not ideal, you could consider two of you flying via CPT, two via JNB on the same day, or even flying out on adjacent days (I realise this is not at all ideal but just exploring every option..!)
5. You might find availability on Iberia, a oneworld partner; this can be seen online by selecting partners, though it is likely this will not be available online until 330 days prior to travel, calling up might have more success.
6. Finally, if you can’t get to the destination you need by being flexible with dates/starting country/class of travel/routings via JNB/Iberia you could consider purchasing some or all of the tickets the tickets exEUR using the ITA fare tool. This only works 330 days out, but in January BA has fares exAMS for £1879 in Club World.
P.S. Don’t forget Virgin, SAA also fly to SA, so if you have mileage in other schemes this may come in handy.
Hope that helps; let us know how you get on!