Such a split distribution is not required to be reported on separate Forms 1099-R, but TurboTax can only accommodate the reporting by splitting the Form 1099-R into two for entry into TurboTax, one for the after-tax portion going to the Roth IRA and the other for the pre-tax portion going to the traditional IRA. With no tax withholding, the details of how you entered this will not be included in your filing, so the IRS will not know that you had to do this workaround to TurboTax's limitation.
