Since you are using the CD or download TurboTax software, first save a copy of your tax return with the first amended return using a different file name, so you don't take a chance of messing up the original file. Open the new copy, go to forms mode, and delete Form 1040X. That will leave you with a dummy 1040 "original" return that includes the changes from the first amended return. Then go back to the Step-by-Step interview and start the amend process over again. The new Form 1040X will then have the figures from the first amended return in column A, and column B with show only the changes from the first amended return to the second.
You indicated that you are using TurboTax Home & Business in Windows, but you posted your question from a Mac. If you need further help or have more questions, please clarify what environment you are working in, which edition of TurboTax you are using (Deluxe, Premier, etc.), and whether you are using TurboTax Online or the CD or download TurboTax software.
