Option 1

After the first user ( device ) has gone through enrollment ( Make sure they use App Based Authentication not SMS ) 

The second user signs into My Sign-Ins | Security Info | Microsoft.com ( Gets the first user to Approve the 2fa ) 

They can then add their device via this method 

Continue through the instructions

Option 2

Set up MS Authenticator on a second mobile device (unsw.edu.au)