Yes, there are several ways depending what you want.
If you are making an animation, where the portal view will be the same all the time, then the easiest way is to render that view as an avi and use the avi as an animated texture assigned to the particular face(s) of the portal object. Import it like any other image texture, but click the “movie” button.
If you are looking for a more dynamic way of doing this, you could use env mapping, but specify an empty as the camera object and move it to the other view.
The first has the speed advantage. The Envmap needs to render 6 views of the first scene, but 5 of them will go unused, since your face has only 1 side of interest. It also allows you to render the other view in another scene or even another .blend file.
The env map will, however, automatically update for changes in the first scene. (if you use the “anim” button).
I’m not sure how well env maps are supported in real-time mode, so YMMV.