The person listed first on the 2018 joint return should be able to use the same account for a 2019 married filing separately return. You will have to make sure that all documents for the spouse not using the account for 2019 are deleted on the 2019 return.
May I ask why you are filing separate returns? Sometimes people don't realize the differences between filing Joint and MFS.
Unless you have a specific reason to file separate returns,
It is usually better to file Joint. Joint has the lowest tax rates and the highest Standard Deduction. And if you are in a Community Property state MFS gets tricky to figure out. Here's some things to consider about filing separately……
In the first place you each have to file a separate return, so that's two returns. And if you are using the Online version that means using 2 accounts and paying the fees twice.
Many people think they come out better when filing Married Filing Separate but they are probably doing it wrong. If one person itemizes deductions then the other one must itemize too, even if it's less than the standard deduction, even if it is ZERO!
And there are several credits you can't take when filing separately, like the
EITC Earned Income Tax Credit
Child Care Credit
Educational Deductions and Credits
And contributions to IRA and ROTH IRA are limited when you file MFS.
Also if you file Married Filing Separately up to 85`% of your Social Security becomes taxable right away even with zero other income.
So you should try to file Joint if you both can agree.
The desktop version can do unlimited returns and efile up to 5 federal for free (IRS limit). State is $24.99 to efile but it is free to print and mail unlimited returns. And it has what if scenarios. You can see and enter directly into the actual forms and you can compare joint vs separate. You can buy the Desktop program here, https://turbotax.intuit.com/personal-taxes/cd-download/