About once every few days on the Amazon KDP or Createspace forums, I see a question that goes something like this:
If I've co-authored a book with another author, how do we go about getting 50/50 royalties?
I can only assume that these good folks haven't reviewed either the Amazon TOS (Terms of Service) or created and logged into their own Bookshelf, at the KDP. Once you've done that, it's fairly clear that there's nothing automatic that can be done about it. One person is the publisher, and the other has to trust the publisher to pay them their due royalties. Amazon will not pay out royalties to multiple people. That's the job of the publisher, which isn't what Amazon is; they are, in essence, your distributor and your storefront. Your retailer. You, as the publisher, are the publisher, and those jobs, along with all those other publishing duties, are yours to fulfill.
This doesn't mean that multiple authors can't all share a single Amazon account--of course they could, if, say, it were some type of publishing cooperative. But the payments will still go solely to the person in whose name the account is held. Period. Don't try to ask Amazon (or B&N, iBooks, etc.) to do the accounting and the payment work for you. They won't go for it.
Bear this in mind when you enter into Publishing partnerships with your friends, or fellow authors. Will your friendship stand the test of auditing Amazon payments, to ensure that you've received your due? Alternatively, perhaps you can set up a Kindle publishing account, into which you can both log-in. At least that way, you can always keep abreast of what's been paid out. Generally speaking, authors, as a group, tend to think that their publisher (or in this case, their retailer) didn't pay them everything that they were owed, so if you decide to jointly publish with someone else, keep this in mind. Set up all your accounts, agreements, etc., to address this almost inevitable scenario.