I would never advise getting involved with anything like this. Let me present a scenario that I experienced many years ago. A female customer was in a very abusive marriage. She had a joint account with her abusive spouse. One day she escaped and relocated to a different part of the country. She called in and changed her address and phone number because she had sole owner accounts with our bank as well and asked for our assurance that we would not disclose her location or contact information to her estranged husband. Don't make the assumption that just being a joint account holder means that you waive your rights to privacy of your non-public information when it comes to your fellow account holders.
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Knowledge is knowing what to say. Wisdom is knowing when to say it.