Sharing strictly between two Family plans, would involve two separate Family Organizational Vaults.
I’m not sure if there is a feature request for the ability to share logins directly between Organizations, similar to a B2B sharing configuration.
Though for your specific need, I believe the availability to add additional licenses to the Family plan would be beneficial.
If that would solve your concern, there is a current feature request for the ability to have additional licenses for the family plan.