Do I need to cite a working paper that was written after I wrote/presented my manuscript?
Asking your editor is a good solution to an unusual situation. You could also include a note at the end of your paper citing the more recent work and giving an explanation that is complementary to the note in the other paper. 

But someone who reads either of the papers would want to be apprised of the other, so the citation would be useful to researchers. 

I think the priority of your work is clear, from what you say, and is recognized by the other authors, so there is no reason to disguise or omit any information.