Welcome to FT jackcarr yes - midnight (or 1am depending on time of year) you call back to add the two Reward seats on to your original voucher! It can be quite stressful if you've got no plan B or can't do any other dates as it's fastest finger first, which can be very sought after on Cape Town, Sydney etc.

Make sure the seats are held instantly on getting through, then explain about the voucher already in play - should be easy enough for them to see. I've had some excellent easy examples, and a coupe of disasters, whereby I had to do the same again 24 hours later! Good luck and report back