Yep, ours were "best friends", slept, played together, to all appearances were inseparable brothers.
We really did try everything. Took advice from cats protection, rspca, bought every product, tried every technique. In the end we happened on a fantastic vet who went through everything again, but in the end was honest- cats are solitary creatures, and siblings are actually in fact worse when it comes to territorial behaviour as they don't keep out of each other's way and respect territory.
Honestly, it's been 10 years since we split ours and it was the best thing we could have done. In hindsight, even though they seemed to get along, they did have a lot of stress related behaviour, including the spraying. Which all went away once they got their own space.
You could try giving them their own rooms, feeding separately, separate sleeping area etc. But in the end we just got to the point that the spraying was so intolerable separating was the last option.