getting one gl object to poke through another

Hi all,

I have two gridshapes that are in the same layer as each other. Object A is flat, like a video screen, and object B has a more complicated topography.

Is there a way so that where the parts of the complicated object B "extrude" more (have greater z value than the corresponding pixel of the A), B is visible, but when its z-values are less than A, then A is visible?

Maybe I'm thinking of it wrong as being an issue with the z-axis. When I look at the objects from the side, it's clear that the flat object is sitting in between the high and low points of the complicated object. Yet either I can only see the flat one or only see the complicated one, depending on how I layer it. I can change the transparency to see them both, but that is not the look I am going for.

Perhaps I will have to make the "lower" pixels have a lower alpha value so the shape is selectively transparent?

I attached a screenshot to better explain.

Thanks for your time!

Adam