I need to file an amended return to change the filing status from MFS to MFJ. Both taxpayers have already filed as MFS, separately obviously. The returns were accepted and processed.

My questions -

1 - Which of the already MFS filed returns do I amend in order to "zero out" the MFS returns and file a combined MFJ 1040X?

2 - Do I use the taxpayer's or spouse's return as a starting document?

3 - Does it make a difference which spouse is listed first on Form 1040X?