No this setup is for security reasons. One postfix is to run on a bastion host and is supposed to relay outbound mail to "CUSTOMERS" ( as in business clients ). The bastion instance should only accept smtp from the smtp server on my internal lan. It should not even accept smtp from any of my internal host except for the internal smtp server. This will prevent spammers from taking control of my smtp server.
A second postfix server will be running on the internal network. It has a two-fold purpose. Namely, to relay mail between internal clients on my network and secondly to relay mail that is internet bound to the postfix instance running on the bastion host. The postfix instance running on the bastion host will then relay mail that is internet bound to the appropriate destination over the internet.
Internet <-- Postfix (bastion) <-- Postfix (internal)
This should not be difficult to understand, since such configs are typical in modern firewalls and DMZs. That's why I find it frustrating that people on the forum would think this is not security related. But I guess that's old hat. So back to your question.
The postfix instance on the bastion simply acts as a internet forwarder/proxy for the actual smtp server on my internal network. In this way the internal smtp server can be isolated from exploits over the internet which may give an attacker access to my internal network where databases and client's personal information might be stored.
This way if the attacker successfully exploits the postfix server running on the bastion, then this will only give him access to an unpriviledged user on the bastion host. If the hackers goal is to get to my internal servers, then he would have to crack into an account on the bastion host with root privs. Then he would have to get through yet another firewall to get to my internal lan.
In closing I must say that what I have just described is extremely security related. And if people on this forum are willing to dismiss such a discussion, which is typical of firewall construction, as , "not being security related", then I guess that why I haven't been able to get an answer to my question.