The license is device-specific. You cannot use the same license in two devices at the same time. If the same subscription key is used when activating a new setup, the license is transferred to the new setup. You can transfer the license to another phone up to three times. After three transfers the subscription key will no longer activate in a new device.
The subscription is also verified during the reputation update check. If you are using the same subscription key in two devices, the subscription key will be locked to the last device after three updates.