You won't get banned, you just won't be able to log both in on the same connection.
An IP exemption will do it, just fill out the forum in the petition section and you'll be safe....proceed with caution if he "steps out to the store for 5 minutes" and a box check just happens to happen at the wrong time.
Also proceed with caution after December, especially if one of you just happens to be able to port or buff - and temptation to quickly log the second account in is too much
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