I have been researching on this topic for a while.
Played the portal games again and again for several weeks. 
The portal team had released several articles on their tech over the years although never really explain how they implemented the seamless physics across their portals. However I came across some papers which explains how the visual effect can be achieved without using render targets.
The idea is to use Portals (not to be confused with portal game portals). Those are the things used in earlier engines to determine visibility of various sections of the map (I think idTech engine and the first UnrealEngine uses it). I dont know if the current generation engines uses it anymore. Anyway inorder to create a portal-game-portal, we will define one Portal (which is a quad) for each opening. During rendering we do this following things.
- render the scene using the active camera. But if the gemetry being rendered is inside the Frustum defined by one of the opening quads (ie the frustum project those 4 vertices along camera look-at direction), we will discard those pixels and do not write anything to the depth buffer.
- In this stage, we will adjust the WorldMatrix in relation to the first opening-portal. So effectively by mutiplying each vertex with this modified matrix, we translate the object that are seen through the paired-opening-portal into the opening-portal in consideration. Then we render those geometry like normal. In efefct those will be rendered inside the qaud of the opening-portal.
- Repeat step 2 for the other portal.
I got a friend to try this with a simple opengl setup (not done using unreal-engine, since we were not sure how unreal’s rendering worked)
The advantage of this method is:
- rendering is straight forward. We are actually translating the full geometry to a new location (not permenantly).
- unreal-engines GPU sprites will still work, and since they test against the dept-buffer for collisions, those particles can go through the portals (but only in camera view).
- Light can pass from camera area into the portal if using forward rendering. But unfortunately not the other way around. But I think with some clever setup this can be achieved. Have not tried using deffered lighting. But seems to me that lights can pass though portals-in camera view atleast.
- Screen-space reflections will show things that are inside the portal.
- No 1 frame lag.
Disadvanatges:
- If we were using render targets we could do recursive portals without any additional work (ie when two portals look at each other). But in this method, we have to re-render objects to get this effect.
- Need to edit the rendering code of unreal. The concept of portal should be supported by the engine itself.
- SceneCaptureComponets may not respect portals.