Random question, since they are both in the Mutsu region. I am wondering if they are maybe physically connected.
If we say the gates are volcanic in origin (could explain the earthquakes) they could be vents of the same passage. This could even link to the malice springs as I call them (e.g. the black water, the rift), as water that has passed through this volcanic vent.
If they are connected, then I am wondering if one was the original entrence and the second opened up after various earthquakes and problems with the original?
I also wonder which of the 2 was more essential to keep closed (ignoring that the hellish abyss has probably been closed forever now).