It has a chart that list a double wall (I assume synonymous w/ room within a room) as having an STC of 59. It lists a single wall of similar construction at 39.
No, unfortunately STC's cant just be added... i don't know exactly how it works (am looking into that now). Will post how they are determined as soon as i figure it out.
I've seen higher STC's in sliding glass doors... also, cheaper alternatives to resilient channels etc, thru a company called www.quietsolutions.com ... I'm currently in the process of building my own studio, and supposedly this stuff is easy to use and the results are just as good if not better than anything else you could use. HAvn't decided exactly which way im going to go... But at the very least, this stuff is an option.