Married couples and those who live in the same household typically file together: One files the I-864 as either the primary sponsor or joint sponsor, and the spouse and other household members file the I-864A.
Joint sponsors are not the spouse or other household member of the petitioner who agrees to sponsor the beneficiary: parents, siblings, friends, etc.
EDIT: Sometimes the issue is related to the browser and any cookies/cached files. You could try to delete cached files/cookies or the incognito/privacy mode of your browser and see it that helps. If it doesn't, then it is probably something NVC has to reset.