Finally get why they need a thousand pages to prove 1+1=2!
The issue is 1+1 has no guarantee it will be two. You look carefully you can see the first 1 is exactly the same as the second 1 !!!!
Hence put the set of all Russell that do that kind of maths and add to another Russell also do that maths. You still ended up with one Russell.
That is why go all the trouble to say no intersection and first oneness set does not overlap with the second oneness set etc etc
Qed