Ideally, you should amend the returns to show 50/50 for all of the years where you were operating the business under this agreement to show two Schedule Cs. The reason for amending, as well as treating it this way going forward, is that you want to each have half of the self-employment earnings on your individual Social Security records for proper calculation of your benefits. It won't change the tax due, but it should shift the credits to the proper Social Security accounts.
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