I only discovered the second portal use in the Dungeon Dragonspire, and have not entered the next dungeon yet, but I would suspect that the next dungeon portal in a seperate dungeon will negate the first one as it does above ground, but come to think of it I have not gone back to a previous map area to see if an old portal is still there...try everything even if the hint books and manual imply you can't. That's the way I found out about the second portal.
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