If you right-click on top of an adjustment brush pin you get a contextual menu with the option to duplicate the brush. This creates a clone of the previous brush that sits exactly on top of the previous one, duplicating the effect (you can edit this later)
As Wayne mentions, there seems to be no way to select the brush that is underneath, besides moving the one that is on top first, as Slobodan suggests. Since the only option to move the brush seems to be to drag it with the mouse, it is almos impossible to achieve perfect alignment on top of the previous brush once you moved it. The brushes do not "snap" on top of other brushes and there is no relation between brushes (like parent-child) even when duplicating one.
Having said that, the exact coordinates of the brushes are saved in the catalog and/or the xmp files, so in theory it would not be difficult to implement the functionality to snap brushes. In any case, I doubt that this will get enough priority at Adobe.
Because the data can be stored in a xmp file, it is possible to reposition a brush in the exact location as it was before by editing the data directly in the xmp file (not something I would recommend doing as a routine). I was able to achieve exactly what Wayne needed:
- Cloned an adjustment brush,
- then moved the new brush to be able to select the original one
- edit the original adjustment brush
- reposition the cloned brush exactly on top of the original one by editing the xmp data.