Another example.
I booked LHR-PVG. This normally costs 110k Avios one way in J plus £250 in fees.
As I booked for one person with an Amex voucher, I got this for half price. 55k Avios.
I rang up later and got the return added for another 55k, so my return journey was half price.
But if I had booked for two people one way, it would have cost me 110k Avios plust two sets of fees (£500).
Then ring up to add the return for two and it would cost another 110k Avios + £500.
Two for the price of 1.
Originally this was the *only* way of using the Amex voucher, it is only recently that you have been able to also use it for one person.

If I had used the Barclays voucher, the one person process is similar. I pay the Avios and fees for W on the way out and back.
But if I use it for two people on the way out, then there is nothing left for the return, you would need to pay the full Avios cost unless you had a second Barclays voucher.
So the two vouchers are very different. For a couple the Amex voucher is twice as valueable as the Barclays one, in this instance you save 220k Avios whereas a single person only saves 110k Avios.