Ah yes, you can't take sides because you never know the whole story. They have to sort it out between themselves.
Focus on the child who is upset, hurt - check they are ok. When DS1 complains of unfair treatment or DS2 behaviour, as he will, then you just have to go back to the I can't police you, you need to treat each other with respect etc...
ds1 (10yo) is currently having LOADS of issues with ds2 (8yo). ds2 has found his 'voice' and this new assertiveness isn't going down well with ds1 who is very bossy to say the least.
Any idea on how to help them find a way to be 'get on' together (as to stop fighting and hurting each other)?
ds1 is at the stage when he will do something to physically hurt his db (like trip him over), with full intent and no regret about it at all. No amount of explanation, finding other ways to deal with it, sending to his bedroom etc... work as 'well he had warned ds2 to do x, he just had to do it!' . Punishment just builds up resentment on ds1 side and tbh, they are usually both at fault (ds2 can be very annoying at times too...).