Is this the offer you are referring to? In order to qualify, you must purchase both devices on EIP and start a new line of service. When you purchase the devices, you may be required to pay a down payment as well as any applicable sales tax for both devices.
Once approved for the offer, you will receive bill credits monthly to offset the cost of one of the EIP on your statement.
As far as your last question, I want to make sure I understand your meaning. Are you referring to a "credit limit" so to speak that would prevent you from purchasing more devices? If you already have an excess of financed devices on your account, you may not have enough available to purchase both devices on EIP as required by the promotion.