Did you already file your taxes? If so, you have to wait until your refund is accepted, then receive your refund, then you have to amend your taxes to add another W-2. If your refund does not get accepted, then you can add your W-2 

To enter/review/delete your W-2 in TurboTax:

1. Open your return.
2. Enter "w2" in the Search box (upper right of the program screen).  
3. Click on "Jump to w2" link.
4. On Review your W-2 info screen, you can click on Add a W-2, Review or Delete (click on “Trash Can” of the appropriate W-2).
5. Answer screen prompts.