I would think in your case you would want to mail it all in one package. If I understand correctly, they want the I-693 medical from you, which needs to be sealed, and a copy of your marriage certificate, which probably should be a certified copy. I suppose you could split the response so the proof of ongoing relationship is uploaded, but I personally would stress out that some jerk might decide that you didn't provide a complete response when only looking at the online or mailed portion.