Is there any chance at all they will sort it out and be ok with each other?
With living animals anything is possible. Plenty of people have multiple roosters and no real drama. Some wind up with dead or seriously injured roosters when they try that. There are several different things that influence that, many that you have no control over. A huge factor is personality. Some are going to fight to the death, no matter if they are losing or winning. Having a lot of room can help, I don't mean 4 sq ft in the coop and 10 in the run. I mean enough room they can each set up their own territory and stay out of sight of each other and each having their own harem. Yet there are some that get along fine in much less space. Each one is different. It seems to help if they grow up together, either as siblings or father-son, but that is certainly no guarantee. Yours have been separated so you lost any benefit of them being siblings. You don't have roosters, you have two immature cockerels really hyped up on hormones. Once they mature enough that they gain control of their hormones things can calm down (or maybe not) but as someone on here once said watching pullets and cockerels go through puberty is often not for the faint of heart. They might calm down some when they both finally mature or they might fight to the death. Yours have shown that getting there will not be pretty.
This is where I usually go through a spiel about keeping as few boys as you can and meet your goals. But I think you have made an excellent decision to rehome that boy.
Was told it was because each rooster needs between 10-20 chickens for themselves and if you have them then they don't fight each other.
Sorry but that is not anywhere close to true. Plenty of people keep multiple roosters with much lower hen to rooster ratios. If the boys are going to fight they will fight over 30 hens as fast as they will fight over a handful.
We have gamefowl and was told that my rooster will fight his son eventually so I'll have to cage him. Up to this point I'm thinking I will have to cage him because I have seen our rooster chase him a bit twice. They sleep in the same pen with the girls and roam free the rest of the day (even during the day they'll lay close to each other without any fuss). But I have no idea if this will last or it's just because he hasn't gotten too hormonal yet...
This can happen with any breed. This thread is about Serama crosses after all and Seramas are supposed to be pretty peaceful. It's not just some breeds. Mature roosters of any breed will often chase their sons when their sons hit puberty and start bothering his hens. There can be exceptions but that is about as normal and standard behavior as you can get with chickens of any breed.
The original gamefowl were developed for cockfighting. They were specifically bred to fight. So fighting and fighting to the death is in their genetics. But cockfighting is no longer legal in any state in the US though people still cockfight illegally. I don't know the legal status in other countries. So there are still plenty of flocks with "fight to the death" bred into them. But there are some flocks of gamefowl where fight to the death has been bred out of them. These can behave like any other chicken breed.
I don't know which flock you have. Cockfighting is still practiced around the world, illegal or not. Once your cockerel reaches a certain point of maturity there is a real chance that it will be a fight to the death. I'd have that cage ready no matter what breed they were, but the risk is higher with gamefowl.
If you want to keep both of the males you might look into staking. That's a pretty common way to keep multiple roosters and stop them from killing each other. I don't know the details on exactly how it is done, how to attach them or how far apart they have to be, but this keeps them from killing each other. This leaves it up to the hens to hang with whichever boy they want to.
I have put him up for rehoming yesterday but nothing as yet. I’m not sure what to do. Top hen keeps making a racket when they fight like she’s telling them off but our cockerel is having non of it! Is there any chance at all they will sort it out and be ok with each other? Excuse the state, we’ve had a lot of rain lol. I had to use purple spray on the “new” one as he got a small peck on his comb before I could separate them. 
