I am drawing two 3D objects, both of them can fade from opaque to transparent independently, and they can intersect between them (so you cannot say when one of them is before the other one). Look at the image for a better understanding (one of the object is a terrain and the other one an area):

Now, if I apply transparency to both of them, and draw the terrain before the area, the terrain is not transparent respecting to the area, but the area is:

And finally, if I draw the area before the terrain, then the area is not transparent respecting of the terrain:

QUESTION: How can I make all the objects transparent to the rest of objects without depending on the drawing order?