Here's a quick and easy way to do this, let us know if it's close enough to meeting your requirement.
Create a parameter and populate it with your Product list from your Product field. This will be the main Product users choose:
Now we'll create a quick dimension to indicate that a Product (in my example it's a Sub-Category) is the one selected, and bring this row to the top. Now, every time the Select Product Parameter is chosen, the chosen Sub-Category is in the first row and the others are below for comparison.
Now for the filter so folks can choose which SubCategories to compare: I created another Sub-Category field that will call out which Sub-Category is the one Selected, then I manually sorted this list so the SELECTED SubCategory is at the bottom.
I'm guessing you prefer this SubCategory not be in the filter at all. I think that to do that and show both the Selected and Compare SubCategories in the same view you would need to union your entire data set to itself. If you need more info on that, let me know. Otherwise this doesn't seem like a terrible compromise .
If you don't need both the selected and compared sub categories in the same view, we could create two separate views and put them on the same dashboard. So with this option, the Compare SubCategory filter will NOT have the selected item in it. The layouts are simple - you can see them in the attached workbook.
Compare.twbx 402.4 KB