I'm in the same boat, have two managed servers on different ports and want to proxy from the same IIS7.5
My symptom is that all requests hit the same iisproxy.dll despite multiple handler mappings to different iisproxy.dll locations.
From what I can gather, I might need to create two application pools in IIS to accomplish this.
From an old oracle document:
"In IIS, set the value for the Application Protection option to high (isolated). If the Application Protection option is set to Medium(pooled), the iisproxy.dll that registered as the first website will always be invoked. In this event, all the requests will be proxied to the same WebLogic Server instances defined in the iisproxy.ini of the first website."
Note the "set the value for the Application Protection option to high" ... Um I don't think that's IIS speak besides its probably referring to IIS6 running IIS5 compatibility or something.
Anyway going to explore creating separate Application Pools though I have no idea what I'm talking about.
Hopefully an IIS guru replies to this thread (appcmd syntax would be welcome