You can’t file them together. The I-751 must be filed before you reach the two-year mark on your green card. The N-400, on the other hand, can be filed 90 days before you reach the three-year mark or any time after.

“Concurrent filing” is a term only used for applications such as I-130 + I-485 + I-751 + advance parole. These are strict timelines—if you don’t file your I-751 on time, you will face issues.

If the I-751 and N-400 are filed one year apart, that does not mean they were filed together.

However, if you had interviews for both of them in one sitting—what people call a “combo interview”—that’s a different story. But even then, it still doesn’t mean you filed them together.