locorebob, what you propose is certainly doable and easily accommodated in TurboTax. It doesn't matter where you obtain the funds to complete in January 2026 the rollover of the December 2025 distribution, the rollover is reported the same no matter where the funds come from. When you enter the 2025 Form 1099-R that includes the December 2025 distribution, you'll simply indicate the amount rolled over. Also, the fact that the rollover deposit occurs in 2026 has no bearing on how you report the rollover of the December 2025 distribution.
If you were over age 59½ at the time of the Roth conversion and the December distribution comes from the same traditional IRA as did the distribution that was converted to Roth, both will be combined on a single code-7 Form 1099-R. TurboTax will likely require you to split this Form 1099- into two for entry.
As you have noted, the January 2026 distribution will be reportable on your 2026 tax return. (The January 2026 distribution will be ineligible for rollover due to the one-rollover-per-12-months limitation on rollovers. You won't be permitted to roll over any IRA distributions made prior to one year after the December 2025 distribution, so you'll need to be careful if you consider doing something similar near the end of 2026. I'm also assuming that you have not done a rollover of any other IRA distributions made in or after December 2024 that would make the December 2025 distribution ineligible for rollover.)
Assuming that you do not annualize income on Schedule AI of Form 2210, all of you income and tax withholding will be treated as having occurred evenly throughout the year.
