Ok, the problem you are trying to solve makes more sense to me now. I'm imagining your environment is like this: You have a web server with one web site configured on an external ip address for public consumption and one on an internal address for your intranet, but pointing to the same physical folder as the external site. You have an SSL certificate on the external site but not on the internal site. Is that about right?
In that scenario what I would do is download the IIS Resource Kit which has a tool for generating a self signed ssl certificate. I would use that to generate an ssl certificate for your intranet (and you can also use it to generate an ssl cert for dev machines for testing under ssl). I can see no downside to encrypting the data whether its on the public or private side. Your private network is probably more secure but this way even if someone gets access to sniff packets on your private network they still can't get the data.
Hope it helps,
Joe