My concern is not with your case where only the after-tax portion is rolled to the Roth IRA and only the pre-tax portion is rolled to the traditional IRA (and nothing left over), resulting in a zero taxable amount. My concern is with different splits and whether or not the rollover was direct or indirect. By attempting to report these with a single entered 1099-R, I am able to cause all sorts of reporting errors in these cases. I suspect that to simplify the program and the question sequence that TurboTax just requires all cases of a single Form 1099-R reporting multiple rollovers to different types of IRAs to be entered as split Forms 1099-R.
Also, because the payer also typically combines multiple non-simultaneous distributions and rollovers on a single Form 1090-R, TurboTax has no way to know (without a convoluted question and answer sequence) if it can apply all of the after-tax amount to the Roth conversion or not.
