Let me try:
We have tried separating them, offering and doing 1:1 activities with each of them (preplanned, not as a reward) but then the second they are together again it is just not stopped.
We separate in our house -- not because it helps with the fighting when they're together, but because there is no fighting when they're not. I enjoy my time with each of my kids 1:1 and connect with them. It may not help the fighting, but it's good for the family overall when they have some time apart.
Larla might say "I like that blue pillow". Larlo says "its not blue its tourqois". I state that is a form of blue. He says "she is still wrong" and she ends up in tears.
I wonder if he would respond to something like: "Larlo, you really know your shades of blue! Can you teach Larla what the different shades are? You're a good teacher and I know Larla is a fast learner."
Larlo will have Larla play Legos but then she will honestly accidentally break something (because 5) and he will yell and scream at her.
And here I would try: "Oh you're frustrated. You were working really hard on that and now it's broken. Let's practice how to handle your frustration in a kinder way." Then coach him to say something like: "Larla, I'm so frustrated that you broke this!" or whatever. Then comfort Larla and say: "I know you didn't mean to break that honey." and just give her a hug.
Larlo will be watching a movie in the family room and larla will bring on 10,000 toys and play in there annoying him (despite having 4 other bedrooms, a living room, playroom,a basement and an entire yard to play in).
In this case, I would say to Larla: "Looks like you want to keep your brother company, but he's concentrating on his show right now. Come with me into the (living room/ bed room / whatever) and let's give him some space." Then I would physically enforce the space. He deserves to have some time on his own and she needs to learn to respect thta.
It just never ever ends. If one says white the other demands black. It is exhausting.
I hear you. My boys are 2 and 4 and are at it all day. It is one reason I was afraid to have 2 kids... I didn't want to be a referee all day long!