Depending on your account status, it is possible to get a second duplicate SIM (that would duplicate a line other than your main one) to use in another phone.
This would only be necessary if the 'other phone' can't run the DIGITS app.
If the other phone CAN run the app, just sign in, choose the line you want to use, and away you go.
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!