OnMouseOver with multiple colliders

I have an object with 2 children : Radius and MouseDetect. They are both sphere colliders. MouseDetect is smaller than radius, and I need to detect whether the mouse is over MouseDetect. OnMouseOver doesn't work in that case (only the outer sphere collider triggers even though both spheres are Triggers).

I tried raycasting with a layer mask on the outer sphere, but it masks the whole object.

Any advice would be greatly appreciated.

Thanks!