I don't fully understand the question. I assume you will write a short letter referencing the N400 interview information, and the I751 information respectfully requesting a combo interview then upload it into the N400 section. Like @Crazy Cat said, it is not certain that will do anything, but it certainly does not hurt to try. Beyond that, I would assume even if you don't upload a letter, I would plan on a combo interview and prepare accordingly with both spouses attending and bringing any additional information you may want to add to the I751 package such as updated tax information.
Good Luck!