Well its been a few months since I posted about the aggression issues we were having between our Border Collie and Ridgeback so I thought I provide an update for those who were curious or in a similar position.
BTW it's a positive update

Max the Ridgy is now 6 months 2 weeks old and and Hugo the Border Collie 5 months and 3 weeks. There has been no escalation in the violence infact its dropping off quickly and we now have two pups getting along very well all most all the time now!
So after my last update we continued walking, feeding them separately and giving each separate time with us. Exercise and mental stimulation at home and away from home also has been very important and helpfull, especially because we live in Sydney with a smallish backyard. Doing these things daily was preventing the issue from escalating, we also kept a close eye on them and have learnt to spot the body language leading up to any aggression so we be proactively address the troublemaker and separate them if using a loud voice to stop doesn't stop them before it escalates!
At its worst it was happening at least once every day or two, sometimes more and it was getting a little worrying to say the least! Saying this there had been no major wounds inflicted on each other apart from a few light scrapes, I did notice some blood a few times but couldn't find the associated cut so attributed it to teething, but nothing major.
Anyway we noticed the biggest change a month ago, we put them both in the kennels for 6 nights while we had some work done in the back yard. The two trouble makers ended up being kept separated due to this fighting (with each other, not others) as the kennels not able to spend the time we do keeping an eye on them. Then the next day after we picked them up we had them both de-sexed.
I'm not sure which helped more, the break in their routine at a new place (the kennels are great btw, and they get alot of out door exercise and they match like dogs up to play together) or the de-sexing. I tend to think its been a combination of both as the de-sexing from my understanding will not make any immediate change if any at all depending on the dog.
So now we rarely see Hugo start up at Max anymore, I'd say once a week at most, starting to lean more towards once a fortnight!
We are keeping up with keeping them separated during different periods of the day but we are now taking them together to the dog park to get a good run. Its a big relief as we would have been gutted if we'd had to pick one to re-home!