Before I get to my question, I know the most obvious solution would be to use the normalized view port rect, however I need more complex shapes than a rectangle, and I've looked in to using the view port rect and it seems to not be my solution.
EDIT: Some people were confused by my question, let me go in to a bit more detail. What's happening is as the player moves in to one portal, I create a clone FPS controller and move it out of the other. This gives me two cameras, and the view you see on the right above. It's just showing one camera, and clipping through the portal. What I want is something similar to this where the cameras blend to create the illusion of a smooth transition. What I want to do is delete everything from the green checker image to the left in the image below, and replace that with the other camera. That way you get the part of camera A's view that is peaking out of the portal, blended with the part of camera B's view that's peaking out of the other portal, to get one complete image. And as you move through the portal the cut changes appropriately.
I've been designing a portal system, I have everything down, including getting the player to smoothly move through the portal. My main problem now is getting that camera blend effect Valve does. I need two cameras to blend together seamlessly, as if you were poking your head through the portal. And it can't just be a rectangle, it has to match however the player is looking through the portal.
My best lead on this right now is to possibly project a depth mask shader behind each portal, then make the camera from the portal you're traveling to be depth only. Then somehow mix the two cameras. My main problem is figuring out exactly how I'd do this, how to make the second camera only render what's outside of the portal, and have the rest default to camera 1 to get one full screen projection.
If you could give me ideas, or explain how I may do this with the depth mask shader that would be a tremendous help. I'll continue to work on this and update as I make break throughs.