On the device with the functional subscription go to the Manage Subscription screen (in the ezccs app, not under device Settings) and press and hold in the middle of TEXT on the screen (red circle in image below). There should be a longish number separated by dashes. Use that number on this same screen on the device without a subscription. After you enter the code, touch Done.
Note that the device the has the subscription you must see a thumbs up and green text.