How to mask a Sprite Mask?

I'm having a problem with Masks. I have a Sprite Mask and a Rect Mask cutting out things from a Scroll Rect (the masks are at the upper side of the red line). Then I'm having a preview box with another Sprite Mask (the blue box) and the Sprites in the Sprite Mask are cut out the right way. But if I put this box outside of the red Sprite Mask the Sprites are still visible. Is there a way I can "cut out" or mask the blue Sprite Mask if it's outside of the red Sprite Mask? All Sprites are only visible inside of a Mask.