Hi Reuven,
I'll try to give some answers to the first question and leave others for more experienced folks here.
1. You install Static pages and General Comments packages using the Package Manager. Remember to restart the server after that.
2. Using the site map create a new subfolder and name it.
3. Click new application, give the new instance a name and submit the form. Again, restart the server.
4. After these steps I had a working instance of Static Pages. I don't see there's a reason to mount more than one instance of static pages, because it's just a way to make static pages commentable and its UI is just for administrative purposes like finding new static pages or to change the commentability of particular pages or folders.
OK, just a bit to the second question:
"Since static-pages is a singleton (and thus site-wide) application, how can I separate the pages on these two sites from each other?"
Static pages is aware of the folders, so if you have your subsites in folders a and b, you can set the commentability prefs differently for the both folders AND for the individual pages inside them.
