I think you're overthinking the problem, it should work fine. Any Real OS (and even the Fisher Price(R) one) allows multiple IPs. If a request comes in on one IP it should be smart enough to send it back out on that IP.

Occasionally you'll have to tell the server program which interfaces/addresses you want it to listen to. You may have confused the programs by adding a seccond IP address and not telling it what to do with it. (See: Apache's "listen" directive)

Matthew