@MJM10
Note that, when amending two separate returns to one joint return, they should pick one of the separate returns (it doesn't matter which) and manually add all the information from the other spouse, including information about taxes paid or refund already paid. It's a little more complicated than just amending without changing the status. There is an article here.
https://ttlc.intuit.com/turbotax-support/en-us/help-article/amend-tax-return/amend-married-filing-se...
They don't need to take any other specific action with respect to the other spouse's separate return, it will be
canceled automatically when the joint return is processed.
