I would just mail two superseding MFS returns and skip the 1040X.  Mark "Superseding return" at the top of each, don't forget to get "ink" signatures.  Certified mail.  If the original MFJ return showed a refund, attach a statement to each return indicating how the IRS is to allocate that refund between the two taxpayer accounts.

My guess is that this will take about 10 months to process but I've actually been pleasantly surprised at how quickly the IRS was able to process the backlog of 2019 returns / unopened mail.  Last I heard there were only 7 million unprocessed returns.

Rick