I had raised this issu over a year ago (with sort of the same thread following): Compare images: how?
There are other threads about this, each with sort of the same result: fellow users don’t understand why some others would need this IN-APP, suggesting external viewers. I sincerely hope DxO will offer this one day. The current “compare” feature is of little to no use. What’s needed is the ability to compare ANY two (or more) DIFFERENT files WHILE edting one.