No, that doesn't work any longer. That's also the page I managed to force.

You can only select one voucher, even if you select 4 passengers.

If you select four passengers BEFORE you click 'use companion vouchers' then the number of passengers resets to two as soon as you select that option. Changing it back to four does not change your inability to use two vouchers.

Tried in Chrome and Edge on desktop and Safari on my phone and no joy.