Why both-at-once matters
Swapping two faces in separate passes means exporting, re-uploading, and hoping the second pass does not disturb the first. Doing both in one operation avoids that entirely — the two swaps are applied to the same original image and land together.
It also removes the most common two-person mistake, which is getting the assignment backwards. You are clicking the actual faces in the photo, so who goes where is explicit rather than inferred from ordering.