Real-time Alpha effect in Eevee Help (similar to Mix node in compositor)

Hey everyone,

I’m doing a test in Eevee, and wanted to try and use a material or object silhouette to drive the transparencies for another material (So like real-time masking/holdouts)

For example, I want to use this material/silhouette:

To be used as an alpha control in this image:

To get this type of result:

I can do it pretty easily with the Mix or Add nodes when in the compositor, but I’m trying to get this affect to happen real-time.

I could have sworn I had a result like this before in Eevee when playing around. But that could have been a dream or something like that.

Does anyone have any idea how to accomplish this?