Use one camera's output as dynamic alpha mask for another camera

Hi, I have been trying to wrap my head around this issue but have not been able to come up with a clear approach.

I want to mask the view of a camera using an alpha channel that is created from a different camera. In essence 3 layers:

Camera one - background. Camera two - renders the scene as an alpha channel - (cull mask set to a specific layer). Camera three - blends with camera one using the alpha mask created by camera two.

Simulated in Photoshop:

I'd post code samples of what I've been doing but I have a feeling it's so utterly wrong it'll just be confusing. Any help greatly appreciated!