You should use a "Top X" filter.
1) First remove the filter on Index.
2) Unfortunately, Top X filters don't work with combined fields, so create a calculated field to do this instead:
[store_name] + ", " + [region]
3) Drag Region/Store to the filters shelf. Flip to the Top tab and set it up like this:
Thanks Ken. I think this might work but I'm running into a sorting problem. Because filtering by Top N does some sort of aggregate calculation across both weeks shown in the view, I'm left with stores in the top N due to their combined metrics across the two weeks. Is it possible to sort only by the most recent week?