Getting a little frustrated with sibling cats...

Question:
 I have two male cats, brothers, that I adopted from a family when they were 3 months old. They're now 8 months old. Okay, so this all started Sunday night. I was sitting on my couch, playing around on my laptop. It's late, probably around ten. Well, I hear this god awful noise from outside (the windows are open due to warm and muggy weather), like two animals fighting/trying to kill each other... and I recognize a voice-- my cat! I jump up and look in my room, where they usually hang out on my bed and wait for me to come to sleep with them, and I notice a screen has fallen out of my window. The cats are nowhere to be seen. Great! So, to make a long story short...er... I spend a couple of hours outside looking for them and calling to them, to no avail. I'm afraid they'll get killed by a fisher. I laid down to sleep, window open, and one cat appears. Woo hoo! Time to wait for the second one. Well... soon enough he shows up at the window... and the cat that came back first lunges and attacks him, making all sorts of horrible noises! Since then, all they have done is fight with each other, and I mean really fight. They have scratches and scabs from their scrimmages. They howl at each other, hiss and scream, and it's driving me crazy. The weird thing is, though, that they can both lay down on the bed with me and be fine, like nothing ever happened. They even sniff and touch each other, and last night one even licked the other's paw. But that's the ONLY time they're not at each other's throats, literally! So... any advice? Please help!
Answer:
 Hi. I can only think of a couple of possibilities. First, are you absolutely positive these are your cats. It could be that a stranger snuck in, and that one of them is not supposed to be there. That's just one possibility. Another is that something happened to them out there, and one of them betrayed the other. Cats are one of the few animals that actually remember and hold grudges, so maybe one turned on the other, and they're trying to hash out their difference. My guess is the second. If that is the case, then make sure they're not ever alone with each other. Keep them monitored and keep a squirt bottle secretly hidden nearby. They probably won't ever go back to where they were before, but don't give up on it.
Answer:
 I thought of the possibility that a strange cat snuck in, but I put collars on them to distinguish them inside the house (both black) and I know it's them. I was thinking that maybe something happened outside, because it sounded like they fought and then when I heard them meowing, it sounded like they were in different parts of the backyard. So, I don't know. They're alone for eight hours, five days a week, while I'm at work. Should I separate them during that time? I'm looking for a squirt bottle to use, also. Thank you for the response. =]