Here's a long article on doing bundles, mostly intended for brand new Leanpub users:

http://help.leanpub.com/en/articles/3287957-how-to-create-a-bundle-of-leanpub-books

The short version is, go here:

https://leanpub.com/bundles/new

On that page, you can create a new bundle. Then, you can add books to it, using each book's URL, and set a royalty split. When the primary author of the other book accepts the bundle invitation, the bundle will be published. They will get an email with the invitation, as well as seeing the invitation here:

https://leanpub.com/user_dashboard/invitations

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