Do you have any ammonia or nitrite? If not then you have enough biological filtration (most likely the biowheels).
If the water is visibly clear then you have adequate mechanical filtration.
That being said you cannot have too much filtration. In a worst case scenario you may find that what was once enough no longer is (for example when the two oscars are fully grown at 15" or so) or if one filter fails for some reason and the other cannot keep up.
I suggest an air stone in every tank. They can save your entire stock one day.