I think the crux of the problem is that you can't use a Barclaycard voucher for one-way flights originating outside the UK (point 11 of the T&Cs) and that having "fully" used one voucher by using it on the outbound for two people, the return flights to the UK would be considered one-ways. You would need to use one voucher for a return for each person, albeit on one booking, which should be possible per the T&Cs point 18. I think the point HfP is making is that it is possible to book 2x one return flight per person rather than two outbound flights for two + two inbound flights for two. I doubt if you have any alternative but to cancel and start again if you want to use both vouchers, unless you can find an agent who is willing and able to swerve around the rules to make it happen given you end up with the same result.

This restriction (i.e not starting outside the UK) is making me rethink whether there is any value to me in these vouchers.