Same Level Category as Layered Navigation on Category Page

Question

I hope I'm asking this properly. I want to allow guests to be able to filter category pages, but by using categories of the same level...for example my setup is like this

-Body Part
--Body Part Sub-Category 1
--Body Part Sub-Category 2
--Body Part Sub-Category 3
--etc
-Pain Type
--Pain Type Sub-Category 1
--Pain Type Sub-Category 2
--Pain Type Sub-Category 3
--etc

I want a customer to be able to be looking at the body part category page and be able to filter by "Pain Type". From my understanding I would need to nest "pain type" categories below each "Body Part" category in order to get this working. Is there another way?

Answer 1

First make a choice for a primary category. (searching by body part or by pain type) These categories should be set to anchored.

Add the other "category" as an attribute(s)(one or more) (be sure to create a dropdown or multiselect to make it filterable in results)