2 portals overlaying each other does not render correctly, shows up as invisible. Portal particle effects still occur and you can still go through them.
Edit: Further testing shows that the graphics settings (Fancy or Fast) and Advanced OpenGL still causes this bug to happen.
What should happen is at least the portal closest to you is rendered while everything behind is not (due to the rendering engine in Minecraft).
Steps to Reproduce:
1. Build 2 portals overlaying each other
2. Stare at it