Originally Posted by Globaliser

This answers the question "how can I access and redeem the second voucher?" with the minimum risk of the booking falling between two stool.s

Of course, this leads to two bookings, one in his name and one in his wife's name. If that is disadvantageous for some reason, and all four passengers must be booked on the same booking, then I think you are right that it would have to be done over the phone, if it can be done at all.