You can, indeed! As long as only one person is using the account, you're welcome to sync your phone and tablet/iPad to your hibooks account. Just make sure that you are logged into the same account on both devices. The best way to ensure this is to go to Profile > Settings on the first device you've signed into, and then take note of the email address or Facebook account that your hibooks account is attached to. Then, log in with the correct email or connect with Facebook on the second device.
Having trouble activating your account on the second device? Find out if you're using the correct login information.