Easy Portal - Blender Shader [Free]
You'll get a blend file with 2 collections: '1 Way Portal' and '2 Ways Portal'.
The easiest and most powerful way to create portals in Blender!
You can also use it to create other cool effects:
1. Add compositor-like camera effects (blur, distort, custom bokeh, etc.)
2. Create screens that show what the paired portals capture
Etc.