How can I blend two rendering group using a custom post processing?

I try to achieve an effect: A group of meshes will be transparent, when the camera is close to them. “Transparent” here means: for this group of meshes, they are opaque for each other, but transparent with other group of meshes; after drawing them into a render target(I assume one render group has one render target, correct me if I am wrong), use a custom post processing to this render target blend with others.
So, my problem is where can I get this render target? How different render targets blend with each other?

Just create one Render target with your list of transparent meshes which are removed from the scene.

Then you can use a BABYLON.Layer in foreground reusing this render target as a texture. It should do the trick.