Can I add a second SSA-1099 form

If the box 2 of the SSA-1099 has your social security number or the social security number of your spouse if you are filing jointly you can add another SSA-1099.  The amount from box 5 from the second SSA-1099 is added to the amount in box 5 that was entered from the first SSA-1099.

Maybe.  Do they both have the same name and Social Security number in boxes 1 and 2?

All SSA-1099 forms get entered in the same place:

1. Open (continue) your return in TurboTax.
2. In the search bar, search for the term Social Security benefits and then select the Jump to link at the top of your search results.
3. On the screen Did you receive Social Security or Railroad Retirement benefits in 20XX? answer Yes and select Continue.
4. On the next screen, check the box beside Social Security benefits (Form SSA-1099), then enter the amounts from your form(s) in the boxes provided.
   • If you received multiple SSA-1099s, add up the amounts for each box and enter them together.
   • Don't include SSA-1099s for dependents. Those should be reported on the dependent's return, but only if they are required to file a return (this is uncommon).
5. Select Continue and follow the on-screen instructions.

If jump to doesn't work, Click on Federal tab, and then Wages & Income.  Scroll down Social Security (SSA-1099, RRB-1099) and click on Add/Edit.