if you amend a return from married filing joint to married filing single, do both persons file a new return

You cannot amend from filing jointly to married filing separately after the filing deadline on April 15, 2025.   And---when you file MFS there have to be two tax returns--one for each spouse.

HOW TO AMEND FROM MFJ TO MFS OR FROM MFS TO MFJ

https://ttlc.intuit.com/turbotax-support/en-us/help-article/small-business-processes/switch-filing-jointly-filing-separately/L3UGzwHa1_US_en_US?uid=m69orsz1

If you were legally married at the end of 2024 your filing choices are married filing jointly or married filing separately.

Married Filing Jointly is usually better, even if one spouse had little or no income. When you file a joint return, you and your spouse will get the married filing jointly standard deduction of $29,200 (+ $1550 for each spouse 65 or older)  for 2024. You are eligible for more credits including education credits, earned income credit, child and dependent care credit, and a larger income limit to receive the child tax credit. 

If you choose to file married filing separately, both spouses have to file the same way—either you both itemize or you both use standard deduction. Your tax rate will be higher than on a joint return.

 Some of the special rules for filing separately include: you cannot get earned income credit, education credits, adoption credits, or deductions for student loan interest. A higher percent of your Social Security benefits may be taxable. Your limit for SALT (state and local taxes and sales tax) will be only $5000 per spouse. In many cases you will not be able to take the child and dependent care credit. The amount you can contribute to a retirement account will be affected. If you live in a community property state, you will be required to provide additional information regarding your spouse’s income. ( Community property states:  AZ, CA, ID, LA, NV, NM, TX, WA, WI)

 If  you are using online TurboTax to prepare your returns, you will need to prepare two separate returns and pay twice since with online, you get one return per fee.

https://turbotax.intuit.com/tax-tips/marriage/should-you-and-your-spouse-file-taxes-jointly-or-separately/L7gyjnqyM?srsltid=AfmBOopGqCNexowW0pYgvsf7ycIkrx4VjO_63UXv6vSnfu3UEGQiKQTh

https://ttlc.intuit.com/turbotax-support/en-us/help-article/income/getting-married-mean-taxes/L2RgmagpE_US_en_US?uid=m69on7t0

https://ttlc.intuit.com/turbotax-support/en-us/help-article/taxation/married-filing-separately-community-property/L11CeLUMs_US_en_US?uid=m69ousyh