Unity Mesh Based Shadows Overlap Issue

I am Making a 2D Game Similar to Rimworld or PrisonArchitect, I have been attempting to remake the the Directional shadow system by making each block that gets placed down produce a shadow mesh.

(Green Tick Picture was Edited in GIMP)

Obviously I want the mesh to be transparent but this causes issues between overlapping shadows getting darker is there a way I can achieve this effect using a different shader? does anyone have any ideas?

http://ludeon.com/blog/2013/08/sun-shadows/

in the comments at the bottom of the blog post the developer of rimworld talks about how achieved this effect. If I could, i would love to recreate that one but i dont have the shader knowledge. Could anyone point me in the right direction?