This sounds familiar, and I have three though a bit younger and, yes, it is easier if there are two of them and one is doing something else.
One thing I do, and I don't know if it is right or not, is if they come to me schreeching that so and so has done this but so and so has done that, I don't get involved if I was not there and haven't witnessed the incident. I just tell them both/all to go to their rooms and come out when they've finished shouting.
It doesn't always work, they don't always go to their rooms without a fight but I am not going to take sides or get involved in an arguement between a 9 and a 7 year old kwim?
Also, I find that if I get out the house for a bit fairly early in the morning it seems to help them get on better than them getting cabin fever on each others nerves.
Dont know if any of that is much use but I am sure you are doing a fab job and what you have said sounds normal to me!
Oh, and re getting them to help, I exchange pocket money for chores, so they all have to tidy their own rooms, they take their plates to the kitchen, clear up any mess they make, set the table etc etc.
Now, my problem with my three is getting them to not sabotage the weekly trip to the shops....