Need Help for constant alpha shader (example image)

I'm trying unsuccessfully a way get two gameobjects whit the same shaderMaterial, to keep the same color if one goes in front of other, they has alpha.

it's hard to me to explaing my problem so a drawn something to help me to explaing

https://s23.postimg.org/49egiierf/Needs_Some_Help_Hereee.png

Hola buenas estoy intentantando que dos GameObjects con el mismo material y con un 50% de alpha cojan el color del fondo pero no uno del otro, osea que esos dos objetos no se sumen su color. aqui dejo un dibujo donde me explico mejor.

https://s23.postimg.org/49egiierf/Needs_Some_Help_Hereee.png