Philmodjunk in this post helped me solve a problem I've been beating my brains out for a couple of days. He suggested that adding a single-line portal, filtered appropriately, and showing the appropriate summary field, allows additional summary fields to be displayed within a portal. Works great!
Here's the next problem: how to sum two summary fields that are within a portal? My initial failed approach was to create a field in the database associated with the parent portal and auto-calculate the sum of summary field for portal A and the summary field for portal B. Here a figure that, hopefully, will make things a bit more clear. Its the green bit that I'm now trying to - so far unsuccessfully - work out.