Actually, you can use window_sum.
In the attached I made a calc for window sum. Another one for ranks of SUM(Count).
And then I made a filter to show the top 2.
In the attached I have the window sum and the rank displayed on the sheet. There is also a [Show top 2] calc on the details shelf.
If you drag the top-2 calc to filters and select for value = 1, (look at the calc and you'll see what it does), you'll see the top 2, and your window sum value remains at 21.
That's because a table calc used as a filter doesn't remove non-displayed rows. The rank=3 and 4 rows are still in the underlying table.
Perfect this is exactly what I needed. Thanks!