How to change alpha on a sorting group as a whole

I have multiple sprite renderers layered in a sorting group

if I change the alpha in each sprite renderer you can see through every layer

instead i wanted to apply the alpha change on the group as a whole, as if it was a single sprite

is this possible or do I need some workaround, like creating a full sprite with no layers?

here is an image of what i want if the explanation was not clear:

 

Pretty sure you'll have to make your own solution for this. You can either have a data struct somewhere where everything from a particular layer is stored and you run through that to alter it, or you can scan all your gameobjects and look for those in that sorting layer (this will probably getting rather slow if you have lots of gameobjects but does have the advantage that you don't have to maintain it manually).