If I file Form 1040-X twice for the same year, should Original Amount for the second filing use the outcome of the first amendment?

When filling the Form 1040X column A (Original Amount) should I use the amount as it would have been after applying the first 1040X ....

That one's right in the instructions for 'Columns A Through C' (emphasis added)

Column A. Enter the amounts from your original return. However, if you previously amended that return or it was changed by the IRS, enter the adjusted amounts.

Also, relatedly, how do I fill in rows 17 and 18? Do I fill in the same values as I did for the original 1040X, or do I add the amounts paid up with the 1040X?

For 16 you should include the payments with the original return and the first amendment; this is noted on the form but more explicit in the instructions for line 16

... The amount you paid with your original return, regardless of method. Also include any additional payments you made after it was filed. However, don’t include payments of interest or penalties, or the convenience fee you were charged.

The amount you paid with the first amendment is an amount paid after filing the original return. This will result in a different total than before in line 17.

For 18 include the refund they computed on the CP12:

If your original return was changed by the IRS and the result was an additional overpayment of tax, also include that amount on line 18. Don’t include interest you received on any refund.

Combined these should result in the correct amount due on line 20.

Although if you get anything wrong relating to payments and refunds they'll almost certainly catch it and correct it. For the things that go into determining (technically 'assessing') net tax -- income, deductions, and credits -- they have varying amounts of information, sometimes a lot, sometimes little or none, so they accept the figures from your return(s), other than clear errors, unless they select you for a full 'audit' (technically examination) which frankly for $2300 mentioned on your other Q they won't. But for payments you made to them and refunds you got from them they have complete, definitive records, and they can override your values if erroneous. If you want to see, request an Account Transcript (it's free).

PS: in future, review your returns for correctness before filing. Many of the IRS' computer systems are very old and you are adding to the strain on them with these repeated unnecessary filings :-) :-)