I'll start with what may be a silly question:

Does the coauthor need to have different info than what is on their profile?
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Typically authors set their author info at:
​https://leanpub.com/user_dashboard/settings
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The use of custom author info is if an author is, say, an expert in two fields (say hiking and programming), and they want to have a custom bio all about hiking for their hiking books, and all about programming for their programming books. But most authors are best served by one bio which goes with all their books.
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Now, in terms of how to set custom author info for a coauthor:
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If your coauthor signs in and goes to

https://leanpub.com/SLUG/settings/custom_author_info
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they can edit their custom author info to be custom for them.
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You as the primary author can do the same for yourself, of course!