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Alternatively, you can click "Add account" at the bottom of this page to add a new account. If you're adding a previously-unlinked account, you'll need to provide the email address as well. Your secondary account should now by signed in and linked to your default account! You can repeat this process for as many accounts as you would like to link.

Summary:
Click your profile image. Click "Add Account" in the drop-down menu. Click the name of an account you'd like to add. Type you additional account's password. Click "Sign in" when you're done.