Make a multisig wallat, 2/2
Print out each keys, two copies each.
Key A X 2
Key B X 2
Give the copy of Key A to a group of people, family A
Give the copy of Key B to a group of people, family B
Dont like this solutions, because it base on people and trust in a way. Bitcoin was made in a way that no trust is needed.
I would use different methods but to secure keys is almost impossible, almost always prone to human errors.
I hope in future there will be more reliable methods including hardware wallets that cant be fooled easily.