There are a couple of ways we could do this. Here is one way using Table Calculations.
I first set up this calculation
[One Category Selected]
This will determine how many Categories are selected. I also added the Category to the VizLoD on the SubCat sheet, and also made the filters apply across both sheets
I add this calculation to the filter in the SubCat sheet and set up like this
Notice the order and the "at the level" setting. Set the filter value to True
Add this calculation to the Category sheet and set up like this
and set filter value to false.
As a single category is selected in the filter this acts a sheet swap and only shows the "right" sheet.
Hope that helps and makes sense