There are 16 slots for instanced uniforms. You can force an uniform to be put into a specific slot with this hint. So basically you "link" together uniforms that have the same slot index, even if they have different names.
However, I think you can't have different slots bound to the same name. The one in the material with the lowest index will take over the slot.
In your case, if you want to control those two uniforms separately, you should name them differently.