When we adopted Frankie, Chris behaved very uncharacteristically toward him. He did whatever he could to show Frankie he was not a welcome cat (See here). Eventually Chris accepted Frankie’s presence and took steps toward making peace with him but unfortunately Chris burned those bridges. Frankie turned the tables on him and became the aggressor and started attacking Chris on a regular basis.
Lately things have settled down a bit and I see Chris’s efforts are starting to pay off. Yes, there are still fights. Some of those fights are even started by Chris, but more and more I see the two of them hanging out relatively close without one trying to whack the other. Perhaps there is hope for the future.