The idea is to create a separate hidable floating panel on the right side of the list and show notes/comments there from the existing items in the list.
Clicking on the note would select the corresponding list item in the tree.
A partly relevant discussion which mentions such a panel: Un-used space on the screen left and right
Would you find such a panel useful?
This reminds me of split pane in vim 
I’m not a fan of two pane outliners, but that’s just me.
I’m not a fan, either
The idea is not about a two-pane outliner, more like a separate comments column on the right side of an outline, like comments in Google doc.
I.e. if you collaborate on a document with people, you can see their comments right away.
Something like this.
Obviously, this is probably not needed when you work alone.