This is a great question! At the current stage of the beta, virtual lines need to be on the same account as the primary DIGITS line. You could however install the DIGITS application on each phone and log in with the alternating numbers. EX: GS7 >install DIGITS > Log in with iPhone number and vice versa. Calls and messages for the "virtual lines" would need to be done through the DIGITS applications.
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!