After you enter the first W2 information, it will ask you if you'd like to enter a second W2. 

If you've already entered one and need to get back there to enter an additional W2, you can get there by the following steps:

1.  Log in to TurboTax Online and click Continue to to open your return (if it is not already open)
2. Click on Federal Taxes, and then click Wages & Income
3. Click on I'll choose what I work on.
4. On the summary page, look for the Wages and Salaries section.
5. Under this, it should list W2 and then Wages & Salaries. Click on the Update/Start button  to the right of this.
6. Follow the interview instructions to add each of your W2s.