It is NOT 2GB for EACH phone, but a pool of 4GB that either phone can access. When the COMBINED usage exceeds 4GB, then you would be paying for overages. This can be 4GB for one phone and 0 GB for the other, 3 GB for 1 phone and 1GB for the other, or 2GB for each phone. It doesn't matter how it is distributed, just the TOTAL usage is what matters.
Edit: Of course, this assumes you are choosing the 4GB tier for $70/month. If you choose the 2GB tier for $60/month, then you would have a pool of 2GB TOTAL data to share.