1. How can I change the owner of an account?
2. Can an account have more than one owner?

I see that active and owner are groups so theoretically there can be multiple keys in either group.

But how can I add a second key to the owner group?

This command fails:

cleos set account permission account_name owner '{"threshold": 1, "keys": [{"key": "EOS_public_key", "weight": 1}],"weight":1}]}' owner -p owner

It fails complaining the keys are not in the wallet or that the wallet is not unlocked, neither of which are true.