The 401(k) plan was correct to issue only a single Form 1099-R. It would have been improper had they issued two.
Because TurboTax is incapable of handing a Form 1099-R that reports rollovers made to two destinations, traditional and Roth, you yourself must split the form 1099-R into two to accommodate this deficiency in TurboTax's capabilities. Splitting a Form 1099-R under these circumstances is a common necessity when using TurboTax.
