you can't, but you can make him an authorized manager provided he is at least 18 years of age.
He can do everything except add/remove/cancel lines.
I don't think two people can be the "Primary Account Holder" (PAH).
The PAH can add an Authorized User, which can make certain changes to the account. An Authorized User cannot close the account.
To add an Authorized User, contact Customer Care.
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