There is no such thing as a sub summary field. There is such a thing as a sub summary layout part and such cannot be used inside a portal.
Take a look at this link for ways to list sub totals in a portal or what looks like a portal: FMP 12 Tip: Summary Recaps (Portal Subtotals)
Thank you. I went with the seperate copy of the table with unique ALIAS fields. It worked like a charm once I set up the relationship.