Here's an update.
After all else failed, I made a tank separator out of Plexiglass. I cut it to the width of the inside of the tank, allowed it to stick out a few inches from the top of the tank, drilled some holes in it to keep the water flowing to both sides of the tank, and kept it in place with some heavy wood on both sides of it sitting on top of the tank.
I put it in place every night a few minutes before the lights went out and kept them separated at night (they never fought when the lights were on).
Every month or so I tested them together for a few minutes at night, but each time then went back to fighting within minutes. So I basically gave up and continued separating every night for months. To get around being out of the house at the time the lights would go off, I just left them together with the lights on.
This went on for several months while I debated how I was going to handle the situation if I went on vacation. I am going away for a few days next month. So last week I decided that after all these months I should test them together again. To my amazement, they are getting along perfectly for over a week now. Not even a hint of aggression. As soon as the light goes off, each goes to the side of the tank they got used to sleeping on and that's it.
So apparently it IS possible to break them of aggression if you get them out of the pattern for long enough.
