This has been discussed - and it can be done. But. The bigger problem than just getting the reference to the 2nd IP into the system, is actually USING it.
SME is just not setup to deal with two external IP addresses. For web stuff, email, ftp etc. it will not make use of the 2nd IP address without significant hacking about in the core of the templating system.
And there's also the firewall issue. You would have to add appropriate rules for the second IP into the Iptables firewall manually, again because the SME isn't prepared for it.
G