Child benefit probably ought not be offered at all, but the criteria there is perfectly simple; does the appointed couple have the required children ? Clearly has no comparison to the tax allowance issue.
It must be considerably easier to check the tax return form and see who was nominated than having to identify the couple and check the ceremony. Anyway one might ask why must they live together in order to get an allowance ? If allowances are going, let couples living apart claim them. It's likely to be more expensive for them anyway.