I am still confused as to how Form 1040-X should be filed to change the filing status from MFS to MFJ.

Allow me to clarify, again.

Since 2 Forms 1040 were filed with a MFS filing status, do we file a separate Form 1040-X for each taxpayer (taxpayer and spouse)? Or do we file only a single Form 1040-X, which makes no sense, in my opinion. Which Form 1040 are we amending then?
Any thoughts?