Yes, this is acceptable. You will use one of your original returns to do so, and only that spouse's amounts will be included on the "Original Amounts" part of the Form. You will enter the other spouse's income and deductible amounts into the now joint return. At the end, you will be asked if you received an additional prior refund or had to pay additional tax. This is where you will enter what happened on the other spouse's return (the amount of refund or amount owed on the other spouse's return). For a reason why you are amending, you can state that the original return was filed as married filing separately and the return is being amended to file as married filing jointly.