It doesn't sound like you need multiple acs instances (were that even possible). It sounds more like you need to look at the host-node mapping section in the admin screens. I think this does exactly what your looking for, it essentially maps differing URLs to different subsites.

In case you aren't aware a single instance of OpenACS supports mutliple subsites, each of which could be separate instances of most applications in the toolkit.

I do remember there were a few issues with the host-node map and things like permission checking, but perhaps someone can remind me if that all got resolved. Don?