It means: "It's another matter. / Not to talk about that here." With more detail, it says: "That incident is not associated with this matter, (so we'd better not mention it now.)" "둘이 싸우고 화해 안 했는데, 이번에 공동으로 작업하는 거야?" (You guys have fought and not settled yet, right? Then how are you co-working with him this time?) "그 일은 그 일이고 이 일은 이 일이다. 일하고 개인사는 구분해야지." (It's another matter. Let us not mix business with pleasure.) (A bit diffrent from "Let bygones be bygones.")

Thank you so much!! Both of you ! You helped me <3

This sentence sounds a lot like "this is this and that is that" so to me, it would translate to "That job is that job and this job is this job". Since 그 = That and 이 = This.

"That job is the job and this job is the job." I don't know if it makes any sense to your purpose.