Amended wife's 1040 to 1040X, transferred my separate income, expenses, deductions. Tried entering my previously-paid tax due to her 1040X (MFJ), but doesn't seem to reduce our tax liability.
Go by the actual 1040X. Line 18 should be any original refund amount and line 22 should be your additional refund. If you paid on your original return it will be on line 16. If there is an amount owed with the amendment, it will be on line 20.
Or what did you file originally? Did you file your own 1040? As Joint or Separate? If you filed Joint you should add her info to your return.
Based on your previous posts (like from July) we are not sure what you have done or filed. Like if you each filed as MFS separately then wait until each return has been fully processed and you got the refunds or paid the tax due on each one. Then you can Amend one of the MFS returns to be a Single Joint return.
How to amend separate returns to Joint
https://ttlc.intuit.com/questions/1899507-how-do-i-amend-a-separate-return-to-married-filing-jointly