Can you clarify if you are referring to the underpayment penalty, or do you want to know how to calculate the estimated taxes?
I realize that I was a little cryptic in posing my question. My specific issue was in determining the prior year's tax liability (for purposes of determining required estimated payments and withholding for the current year) for each spouse when they filed single in 2023. I was unable to find anything specific in the Community or TurboTax help, but I believe that it is as simple as just adding the two prior year taxable incomes together. TurboTax is pretty oblique on this -- the input of the prior year's actual tax liability is a manual entry in the Federal Carryover Worksheet, P2, Line 6. In transferring this amount to Form 2210, TurboTax applies the 110% "safe harbor" in the background. Everything then flowed correctly through the Form 2210.
Thanks for responding, but I believe that my question has been answered. Maybe this will save time for someone else.