Well we've been leaving them alone now. Still no Feliway.
There's been the occasional screaming followed by a guilty looking, fleeing Luke. But nothing major.
Last night, I heard some hissing and growling and turned around from the computer to see Flash in the kitchen. (A room where she normally steers clear of. (Luke and Leia territory.) She was cornered in the kitchen and hissing and growling at Luke who did nothing but sit there and look at her. Every minute or two he'd get up and carefully walk a little bit closer (curious like) to Flash. She'd back up into the corner and hiss and growl.
I tried to take a video of what happened next but my camera had some weird write protect error and wouldn't let me do it. The fight that happened moments after these pics was pretty intense.
Leia had moved over to the kitchen to see what the commotion was all about and was blocking Flash's escape route. When Luke had finally backed her as far into the corner as she could go Flash started swinging wildly.
Luke would then do the same. When Flash stopped swinging they'd have a stare down and Luke would just sit there looking either bored or curious. When Flash seemed to be about to start swinging again Luke raised his paw as if to swat at her but never followed through if Flash wasn't swinging. At the peak of the confrontation Luke snagged a few chunks of fur out of Flash which brought the fight to a stop as they both watched the chunks of fur floating through the air (Girlfriend and I laughed hartily at their A.D.D.) Luke had a big clump of fur trapped in his paw under his claw which he walked a few feet away and promptly pulled out with his teeth and ATE
as if it was his favorite treat! Luke then sat down at the entrance to the kitchen and ignored Flash who still wouldn't leave because she was to scared to walk past Luke. I decided to call to Luke to get him away from the entrance so Flash could leave. It worked.
We checked both cats over for damage but neither had any we could see.
Today I got the Feliway. I bought both the plug in and the spray bottle. We'll see if that helps.
Took a lot of restraint for us to avoid breaking up the fight but we did it.
PS. I had a thought.... My girlfriend has been out of work since we moved and has been at home most of the time. If the apartment was vacant more often, with no-one to intervene, the cats probably would have settled this a long time ago.