Sorry Online is only 1 return per account. If you change something you can't go back. It automatically saves every entry you make. You can download the .tax2020 file to your computer and open it in the Desktop CD/Download program. Then you can copy it to do test returns, etc. But you can not copy it back to the online version.
You can only switch one way from Online to Desktop. Not from Desktop to Online.
To compare Joint to MFS. If you are using the Online version, do NOT change anything on your return. You would have to start with a new account and do a test return. You don't have to pay unless you want to print it out. So you might need 3 accounts, one for Joint and two MFS, one for each spouse.
How to Compare Joint to Married Filing Separately
https://ttlc.intuit.com/community/married/help/how-can-we-compare-married-filing-jointly-with-marrie...
How to start another return in the Online version
https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-start-another-return-in-turbotax-onli...
It would be better to use the Desktop CD/Download program. It can do unlimited returns and has a What If worksheet to compare them and many other advantages over the online version.
You can buy the Desktop CD/Download program here
https://turbotax.intuit.com/personal-taxes/cd-download/
