Hello,
Is it possible in blender to have 2 objects, and 2 light sources. Have Light source 1 ONLY cast a shadow on object 1, and have light source 2 ONLY cast a shadow on object 2?
I just learned how to make an object invisible but you still see the shadow, but how would I make it so that it would only cast a shadow from a specific light?
Is this even possible?
No.
This is commonly known as light linking or including/excluding.
I believe it is supposed to be implemented into Cycles X which will be released in December with Blender 3.0.
Not sure if light linking is going to be in there yet but IIRC a developer mentioned it on devtalk.blender.org .
Ok awesome thank you so much! That at the very least gives me a starting point on what to search for. I do hope they implement it soon. I can wait a few months for that.
Even good ol’ BI had “layer-specific lights.” And they came in very handy.
It will be great when Blender gets “light linking or including/excluding” but Blender does still have “scene specific lights.”
EDIT: I have corrected these are “SCENE” specific lights not “LAYER” specific.
Each scene can have its own lighting setup. With a shadow catcher and a bit of a workaround you can do something like what you are after.
Here I have 3 scenes composited together.
One with a ground plane, a cube, a cylinder and 2 lights (whithout shadows)
One with a shadow catcher instead of the ground plane, the cube and one light (with shadows)
The last with a shadow catcher instead of the ground plane, the cylinder and one light (with shadows)
I included the cube and cylinder to the first scene (with the ground plane) to be able to fade the shadows a bit with the factor of the alpha over node.
In this version I changed the lights from sun to spot and gave them different colours so you can see how each light contributes to each object.
Holy crap this is PERFECT! Thank you! Gonna spend the next couple days figuring out how to do this! 
Pleased you like it 
I know it is not as easy as a light linking system but it does have many possibilities.
For example if you want both lights to fully illuminate the cube but only one of them cast a shadow you can easily copy the red light to the cubes scene (a new copy-not linked) and turn off the shadows for it.
You do not need the cube and the cylinder in the ground scene, you could adjust the intensity of the world background to make the shadow softer. To do this you have to give the cube/cylider’s scene unique world settings.
Here is the file if you want to take a look (hit render in the ground scene):
Shadows.blend (129.2 KB)
I am not sure, but I think E-Cycles has light linking.
And it also has light groups which lets change light colors and brightnesses and stuff like that even after rendering.
I have never tried it so I do not know good it works but I think it is an option in E-Cycles.
Thank you so much! I really appreciate it! This DOES help me, but I’ve never touched the compositor before, so I don’t know what I’m looking at yet, so I need to dive into how it works, which is actually great because I’ve been needing a reason to learn crypto mattes, and this is going to complement that. I really appreciate you uploading that! 
