SQL Servers Communicating Across A DMZ

In a proposed architecture, a production SQL Server will live in the DMZ, and an additional internal Production server will reside on the internal network. If Port 1433 is opened on the firewall and access to the DMZ SQL is restricted to only the internal SQL server, is this secure?

I've been told two stories:

1) That if port 1433 is opened, that a hacker can enter the DMZ and then browse ANY SQL Server on the internal network, not just the internal SQL server mentioned in the proposed architecture above.

2) If you define a rule on the firewall and you use a trusted connection, then ONLY those two SQL servers can communicate with each other and that it is secure.

Which is true? Are there other methods necessary to allow secure access between the LAN and the DMZ when two SQL servers need to talk with each other? Is allowing this kind of direct SQL communication considered to be bad practice?

Thanks, in advance, for the assistance.