[CentOS] Two Instances of Apache; Primary IP / Secondary IP

Tried the apache group, and no response.  Thought I'd try here.

I have set up a development environment so that it mimics a production
environment.

The production environment has a proxy server (apache 2.x) that sends
requests onto another back-end apache server, and of course the proxy
server serves up pages sent by the back-end.

All that works fine.

However, in my development environment, I thought that I'd set up a
separate instance of apache on the same server, and add a secondary IP
address on the same interface.

The proxy apache instance listens on the secondary IP, and the
back-end instance listens on the primary IP.

When I send a client request to the proxy, I get a blank page.  When I
check the logs, the back-end shows requests coming from the primary
IP, and not the secondary IP.

My conclusion is that the proxy is sending its outbound traffic on the
primary IP address, not the secondary IP address it's listening on.
That in turn means that back-end is sending its pages back on the
primary IP instead of the secondary, and that means the proxy instance
isn't receiving answers to its request.

I know that BIND can be configured to send requests on secondary IP
addresses.  Can Apache?

Is there something I can do with routing tables that can help?
  === Al