Hi @lglen2 ...
You absolutely can use the same rubric for two different assignments. But, as you have seen, if the rubric is attached to two or more graded assignments, then you cannot use the pencil icon to edit the rubric (well, you can, but I believe it would create a new rubric based on the one you are editing). You have to remove the rubric for all but one assignment. Then you can edit the rubric. Finally, you have to re-attach the rubric to those other assignments. I agree...it is a clunky process...but that is how things currently work in Canvas.
Have you checked (and re-checked) every graded assignment in your course? This would include regular assignments, graded discussions, and graded quizzes. There's not a great way to determine where you might have attached the same rubric to another graded assignment, discussion, or quiz, unfortunately.
Keep in touch with this...thanks!