The quick and dirty approach would be an ssh tunnel or vpn. There's also GRE tunnels which would be faster, but not encrypted. Any of these would work without having to change the application.

Probably, though, it's best to start with why you would want to do that. You could be, for example, trying to solve something where a pub/sub model would work better. Or just two separate apps, on different ports. What's driving the idea of multiplexing?