If you expect to receive a Form 1099-B (Proceeds from broker or barter exchange) from your stock broker or investment firm, then you should wait to make your Schedule D and Form 8949 data entries until you actually receive that document.
If you know in advance that you won't be getting a 1099-B for this particular stock sale(s), then you can safely divide your original cost basis between the (2) "new" companies, such that it exactly totals the cost basis in your (1) "old" company. The company that completed the spin-off (or split) should, probably, have some information to this effect on their investors' webpage.
In any event, you technically should make (2) sale entries on your tax return to support this sale. However, as long as you sold both "new" companies during the same holding period (long-term vs. short-term), then there really is no income tax difference at all as to how you report it, and it could done as a single entry.
Your actual determining factors, for figuring your capital gains (or losses) will be the original cost basis of your "old" stock (however you allocate it, or not, between your new shares) and your net proceeds from the sale(s).
Perhaps the following TurboTax webpage will be of some further assistance in understanding:
https://turbotax.intuit.com/tax-tools/tax-tips/IRS-Tax-Forms/Guide-to-Schedule-D--Capital-Gains-and-...
As to how you mechanically begin the stock sales entry sub-interview in TurboTax, you can do the following:
1. Log into your TurboTax online account and begin your return.
2. Find the Search box in the upper portion of your screen, enter the search string "stock sales" and select the Jump To option that should appear.
3. This will take you to the stock sales initial interview and data entry screen
If you encounter any difficulties with the above, please feel free to contact us for help from a live agent:
https://support.turbotax.intuit.com/contact/
