Well there is a module for apache that can function as a sort of load balance mechanic, but i don't think that's what you're looking for.
I assume both servers run completely different domains, if so, then both need a WAN IP to talk properly to the Internet and serve their services to it. There might be a way that I'm not aware of.
Server A: 192.168.1.1
Server B: 192.168.1.2
Router: <WAN IP>
On your router you say:
"If a request comes on port 80, go to <IP>"
That can either be Server A or B .. not both
Server A: apache / ftp
Server B: mail / dns
will be no problem, since all services use different ports.
How many domains your running per server? I have 5 ISPConfig servers running, one of them easily runs about 60 domains. (it's a AMD Opteron 2.2GHz with 4GB mem running on a XEN installation (image based) and on a DRBD disc for HA).