The app is useful for a couple situations.
1) On a device without a sim (wifi-only tablet). It allows the device to send / receive calls and texts over wifi.
2) On a device with a sim (on either T-Mobile or another carrier). It allows you to keep the primary number on that device available for calls/texts and the app for calling / texting with your DIGITS number. This allows you to have 2 numbers on the same device. (Say using a personal device for both personal and work lines).
In your case, you don't have another line of service on the duplicate phones to worry about, so the app is not necessary as the you are using the duplicate sim card.
Was on the phone with support at midnight and that's the answer
Right now one sim duplicate not doing anything, the other went rogue and took over main line - support is working on that
This thread was the product of our fantastic collaborators during DIGITS Beta! As many terms have changed and bugs have been exterminated since the Beta launched, the information here may no longer be accurate. If you still have a question, please search our active threads or ask a new question in the DIGITS space!