What are you optimizing your environment for here, other than the fewest possible boxes and the fewest IP addresses and probably the lowest cost? Where do you want to get to?
You can't "fork" IP packet traffic and perform the requested load-balancing and the single web-facing static IP (this requirement from the early part of the discussion thread here) without some sort of load-balancing, or without a reverse proxy. The IP address has to be routed to a network box somewhere, and that box somewhere has to decide where the packet should be forwarded when you're seeking for load-balancing or failover.
If you have two sites and two public DNS domains and you want that to go to one static IP address and then load-balance to two servers, then you need some add-on network hardware. That load balancer can be a Linux box or a commercial appliance, running nginx or any of various other software. That load-balancing box accepts the incoming IP traffic, peeks at and finds http traffic and then the target domain, and forwards the request appropriately. As a general rule, IP routers don't peek that far into the network packets.
Now if you have two domains and two web sites and two public DNS translations that you want both going to one IP address and one host server, that's feasible. That's virtual hosting.
Or if you want one box to forward some web requests through to the other box, that's entirely feasible, but that also tends to be a case of "why bother" as you're loading up two boxes, and your web environment is entirely dependent on the "reverse proxy" box accessible and working. Mac OS X Server does have reverse proxy, which is what can have one web server serve through contents from another box.
Or you get a couple of more static addresses.
Regardless, Mac boxes (and most any other box running a multi-processing operating system) make for very expensive and slow IP routers.
