Legally now an individual is only allowed a single active number so the NI number itself can be changed. Its up to the individual to apply for a new number and once given the number active use of the old one would be fraud.
The two numbers should be linked on HMRC's systems.
so if an employer had 2 companies (banks will) and so had 2 (valid) Er numbers running payroll for both ee numbers (one valid one not) would result in the HMRC system calculating a taz bill for the ee.
All od that is only possible due to the computerisation of records.
Historically women who did not work outside the home would never have been given a NI number and would have been given her husbands NI number and even been allowed to pay reduced NI
https://www.gov.uk/reduced-national-insurance-married-women
So if old enough a married woman could be issued with her own NI number and have NI paid under it plus under her husbands plus been given a new number if she went back to work as Mrs Husband's-NI-No
Computers allow a quickly adjustable personal allowances so the Bill would be round to arrest you and put you and your slick looking slax in jail.
Now if you end up in girl jail or boy jail that a whole other debate.
Irelands tax case law is a reflection on how the attitudes to women in the workplace has changed. There were test cases which are easy to find of how married women had to sue the Revenue commissioners (HMRC) to be seen as individuals
It has married womens act about debt
https://www.irishstatutebook.ie/eli/1957/act/5
but the husband was legally liable for all the tax and PRSI (NI ) up to the late 80/ early 90
Even today married couple can opt for joint assessment (one liable person) or seperate assessment (each liable)
I dont think men can get Irish child benefit without the mothers permission.