If you already filed your Federal return and received a refund (a refund is the amount you receive back; a return is the form you file), then you should be able to amend it. See this article and this one for more information on filing an amended return with TurboTax.
Form 1040-X, which you file to amend your Federal return, just became available in TurboTax in the last few days. Check to see if you are using the most recently updated version of the software. See this help article for more information.
This older TurboTax tips article on RDP still has some good information.
You actually prepare three Federal returns but file only two of them as Registered Domestic Partners. The creation of a “mock” federal return that reflects joint-filing status is just to get the numbers you need to fill in the sections of your state return that use numbers from a "married filing jointly" federal return. This ”mock” federal return never goes to the IRS; you submit it with your California state return.
Each partner in an RDP includes income from their separate property on their own 1040. Using the Allocation Worksheet from the IRS's community property publication 555, you can establish your shared, or community, income for each category of income such as wages, dividends and interest. Divide the totals in half to give each of you 50 percent.
See here for California FTB information on married or RDP filing separately.
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