Occlusion Culling for Portals

Hi! So this is my first time posting here because I couldn't find anything that can actually help me.

I have a seamless portal system that works normally and some techniques to optimize the rendering in portal camera's. One of the techniques is called frustum culling. The problem is that frustum culling doesn't check if portals are blocked by other objects and can not be seen because of that. I think that not rendering portals in player frustum but occluded by objects would help my game a lot in performance.

I can't use unity's occlusion culling because I can't just remove the portals that are occluded, since the plane of the portal is rendered by the linked portal camera, not iself. So for example, there's portalA and portalB. If portalA is hidden and in player's frustum, portalB stops rendering portalA. But that doesn't mean that portalB is occluded as well, so if portalB is seen, portalA needs to render portalB, and that explains why I can't remove any portals, only stop manual rendering by script.

That said, I really wanted to know if any of you can point me in the right direction (articles, books, videos, ...) to solve this occlusion culling problem. I have read that some people use raycasting but it didn't work for my case since it's a plane and it can be very large.

Thanks!!