The pattern
One person should not be the backup.
In most homes there is a keeper: the one who set up the router, chose the passwords and can be phoned at work to read out a code. It works until they are unreachable.
A shared vault fixes that without turning into a shared account. Both people know the password; each holds a card. There is no invitation to accept and no permission to revoke.
It also means agreement is physical. Changing what the household keeps means sitting down together and rewriting the cards, which is a healthier ritual than one person quietly editing a list.
Arrangements
Three ways households use a set.
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I
Both, everything
One shared vault, one card each, one card at home. Simple, and appropriate when everything is genuinely joint.
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II
Shared plus private
One card for the household vault, and separate cards for each person's own secrets. Nothing forces them into the same vault.
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III
Family, later
A card and a sealed password left where an adult child or executor can reach them. Explained in advance, not discovered.
Shared, without
sharing an account.