avoid multiple focus on overlapping buttons

How can i avoid multiple focus on overlapping buttons? Is it possible?

Here my example: the scene contains a SelectionGrid with a large number of elements and these elements exceed the space available in the scene. Now, to make the list of elements easily readable from the beginning to the end, I need to create two fake scrollbuttons that move my list up and down. The problem is that when I try to click the scrolbuttons I create, the SelectionGrid buttons get clicked as well. I've tried everything but cant solve the problem.

Thanks in advance.

According to my little experience so far, you can solve that using two scripts.

class FirstScript (MonoBehavior):
  def OnGui:
    GUI.depth = 2
    # paint your selection grid

class SecondScript (MonoBehavior):
  def OnGui:
    GUI.depth = 1
    # paint the fake scroll buttons

With this solution the occluded buttons do not react to clicks, however they still seem to show the roll-over effect. Is this a bug? It is difficult to tell from the scant documentation on UnityGUI Events.

Checking the depth of the control could work? (not sure)