In the case were there is after-tax basis in the distribution from the 401(k), I'm not sure that modifying the single TurboTax 1099-R form in this way results in the proper allocation of the basis.
Just be aware that if changes made directly to the form in forms mode produces any related calculation error in TurboTax, TurboTax's accuracy guarantee does not apply. Therefore, TurboTax instructs the user to split the Form 1099-R, preserving the accuracy guarantee (and ensuring the proper allocation of basis).
