When I choose the Diff with active file (when right clicking on the tab header) in the new, when I paste content in there, it edits both the original and the new. Just a note with this, I am using an unsaved file, not opening a file from the hard drive. This is handy if I am pulling in text from elsewhere to do a quick compare.
When I right click (in the file text) and choose split diff > toggle, it all works as expected.
I am using atom 1.34.0 x64 with split-diff 1.5.4.