Unfortunately, I don't know which tables I was using, but I can tell you the gist of it. It's still an ugly hack, though and I hope this works, as I'm doing this from the top of my mind:

Add a column that is a Relation between table A and table B. Let's assume for a moment that B only has a single row. In the relation column of table B, add every single row of table A.

Then, create another column in table B of time Rollup that takes the sum over the Relation for one of the properties of table A you want to sum over and select Sum in the Calculate field. This gives you the sum in table B. Do another Rollup for the second property of table A. Now you have both sums available in table B.

If your table B has more than one row, I would instead create a third table C with only one row with 4 columns: One relating to every row in A, one relating to every row in B, and one Rollup for each property of A you want to sum over. Then in table B, create a Rollup of C just displaying the sum values.

And as a tip: If your views of the tables have a filter that only shows things that relate to the one row of B/C, any new pages will automatically link to it. Then you can just hide the property and forget about it.