Please note: there are two Feliway products. The Multicat would be most effective in your case. I wouldn't belabor the point but I really believe it helps and I've been in your situation and feel/understand your frustration.
I do have a bit of an update. We had to go out last night, and upon returning home I went into the basement office. Both came down and wound up sleeping a few feet apart for the entire night.
Today, Aggie ran up the basement steps and around the sofa...not knowing Allie was on the other side. I think it startled them both and a bit of growing ensued. I watched from a bit of a distance as I was on the phone but it quieted down. A bit later, both were in the office again and I heard some growling. They wound up 6 feet apart with no further issue.
As I side on the floor now (using the slot car entertainment) they are 10 feet apart.
Looks like the growling quiets down quicker now....perhaps we're making some progress.
I would consider less and shorter amounts of growling an improvement.
It is great that they are sleeping a few feet apart two. You are doing things right but the cats will resolve their issues when they are ready to. As a human it can feel like things are taking forever but time works differently in the mind of a cat than it does in the mind of a human.
My 4 cats for the most part get along. I have a female and three boys. My can peacefully exist with everyone but wants to play on her terms. If she does not start the play she will hiss and growl. No one hisses or growls back at her. No one goes after each other. Starbuck's hissing and growling is simply her communicating that she does not like something. She will play with my other three cats and groom them but sometimes she does not want to be bothered. I think this is what you are seeing with the growling and hissing between your girls. When we are home Starbuck will sometimes run to us to get away from someone while she is hissing and growling. Once the offender goes away she goes back to whatever she was doing before. You are definitely seeing improvements even though it may not feel like it to you at times.
Yep, this is real progress. A slow improvement is often the best one, you know. We measure these things in inches rather than miles, and they are doing well! A little growling is nothing at all, after all.