If you have multiple devices in your household, you can login using the same account to be able to control each ilumi.
Once an ilumi is paired with an account, it can't be paired to a new account unless the ilumi has been manually reset. This is to keep your ilumi from being able to be controlled from devices that you don't approve. Like the Johnson's next door!
Please note: If you create an account with Facebook, you'll have to login with the same Facebook account on your other devices in order to access your accounts from multiple devices.