The crude diagram below might help. If I select X from the top matrix, Location: Atlanta and Customer: B are selected. Selections apply to the bottom matrix and histogram.
If I select Y from the bottom matrix, Item: Widget X and Color: Red are selected, but now Location and Customer are cleared. Further, any page level slicers (not shown) never appear as being selected.
The objective is to make selections from the top and bottom matrix, and have all FOUR filters apply to each matrix and the histogram on the right. (Yes, alternately the user could just pick the page level slicers, but this particular report provides insight by exploring the top matrix and seeing the bottom matrix and histogram updated, followed by making a selection from the bottom matrix.)