Really stinks at that happened after 3 years. I would have to think that nobody is safe if they can be peaceful together for 3 years and then turn Rogue. Hopefully, that won't happen to me. I'll take my chances. I appreciate the input thoughI bought what I was told were a pair about 25 years ago and they swam around my 1000 litre system for around 3 years then one morning I got up I found one constantly attacking the other until I removed it and put it in the sump temporarily but it sadly passed away shortly after.
So if I was you I wouldn't put another into the tank where one already claims it as their area. Buy you could try put both in a qt at the same time with the tank split into two and see how they get on then you could try them in the same tank together.