From the IRS instructions for form 8949 (the wording assumes the entire 1099 amount was for the other party)
You received a Form 1099-B or 1099-S (or substitute statement) as a nominee for the actual owner of the property: Report the transaction on Form 8949 as you would if you were the actual owner, but also enter any resulting gain as a negative adjustment (in parentheses) in column (g) or any resulting loss as a positive adjustment in column (g). As a result of this adjustment, the amount in column (h) should be zero. Enter code N in column (f). "
There is no provision, in TurboTax (TT), for this, that I can find. There are several workarounds you can use. Since the IRS gets a copy of the 1099-B, you should not change the gross sale proceeds amount. The simplest thing to do is adjust your cost basis to reflect the amount that went to the other party. Increase your cost basis to reduce your capital gain or lower your cost basis to reduce a capital loss.
At the 1099-B screen, in TT, click on the "I'll enter additional info on my own" link. On the next screen, enter "corrected cost basis". Using an example: you sold the stock for $10,000 with a $4000 cost basis for a $6000 capital gain. Your share of the gain is only $3000. Enter $7000 as the corrected cost basis. TT will enter code B, rather than the proper code N.
If you have download (or CD) software, Report the 1099-B as shown. Then, in the forms mode, use the override feature, on form 8949, to enter a negative adjustment (the amount of the gain to go to the other person) in column (g) and code N (for nominee) in column (f). The override will probably prevent e-filing. Override is not available in on-line versions of TT
Alternatively, you could add the "expense of sale" adjustment, in the interview to deduct the amount going to the other party. But TurboTax will not correctly code that as a nominee deduction.
You should probably consider mailing your return and attaching a statement to your return to explain the adjustment and who the other party (with SSN) is.
