Wow thank you!! Really nice didn’t expected an answer that quick 
I guess your posted Links deal more with the actual “Portal Game Portal Idea”.
I found this though. It comes really close to the effect I want but it should be “two way” so you have to be able to get back through the same door.
Maybe level streaming is the solution, would be cool to find something related.
I try to upload the Stanley Parable reference later.