My grandma's house has 2 wooden doors on one doorframe. I once checked the surface temperatures during very cold weather to see how they insulate, but I can't find the data now. I do know that on summer mornings when the (east-facing) door is in full sunlight, the inside surface of the exterior door gets quite warm, but the interior door is still at room temperature.
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Originally Posted by NeilBlanchard
I don't know about exterior door frames, but I know that arrangement is used in hotels as the connector door between rooms.
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That may be acoustic insulation. Also,each door would have its own lock and passage would be possible if both doors are unlocked.